List of The Big Bang Theory characters
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The series initially centers on five characters: roommates Leonard Hofstadter and Sheldon Cooper, two Caltech physicists; Penny, a waitress and aspiring actress who lives across the hall; and Leonard and Sheldon's friends and co-workers aerospace engineer Howard Wolowitz, and astrophysicist Raj Koothrappali.
Over time, several supporting characters have been introduced and promoted to starring roles, including Leslie Winkle, Amy Farrah Fowler, Bernadette Rostenkowski-Wolowitz, Stuart Bloom, and Emily Sweeney. The series also features numerous supporting characters, each of whom plays a prominent role in a story arc. Included among them are parents of the main characters, their dates and their coworkers. Celebrities such as Stephen Hawking appear in cameo roles as themselves.
- 1 Character appearances
- 2 Original main characters
- 3 Additional main characters
- 4 Recurring characters
- 4.1 Alex Jensen
- 4.2 Arthur Jeffries
- 4.3 Barry Kripke
- 4.4 Bert Kibbler
- 4.5 Beverly Hofstadter
- 4.6 Claire
- 4.7 Dan
- 4.8 Dave Gibbs
- 4.9 Eric Gablehauser
- 4.10 V. M. Koothrappali
- 4.11 Mrs. Koothrappali
- 4.12 Janine Davis
- 4.13 Kurt
- 4.14 Lucy
- 4.15 Mary Cooper
- 4.16 Mike Rostenkowski
- 4.17 President Siebert
- 4.18 Priya Koothrappali
- 4.19 Ramona Nowitzki
- 4.20 Ruchi
- 4.21 Wil Wheaton
- 4.22 Mrs. Wolowitz
- 4.23 Wyatt
- 4.24 Zack Johnson
- 5 Minor characters
- 6 Guest stars
- 7 Notes
- 8 References
- 9 External links
Original main characters
Leonard Leakey Hofstadter (Johnny Galecki) is an experimental physicist with an IQ of 173. Originally from New Jersey, he received his PhD at age 24, spending at least some of his time at Princeton University. He shared an apartment in Pasadena with colleague and friend Dr. Sheldon Cooper until season 9. Unlike his equally geeky friends, Leonard is interested and quite adept in building interpersonal and social relationships with other people. Compared to his friends, he is relatively successful with women.
The writers have toyed with a romance between him and next-door neighbor Penny since the series premiere, with their unresolved sexual tension being a major force for drama. Leonard dated Penny for most of season three, although they had dated briefly at the end of season one. Aside from Penny, Leonard has had romantic relationships with co-worker Leslie Winkle, physician Stephanie Barnett, an unnamed French literature PhD, and North Korean spy Joyce Kim. Following his break-up with Penny, Leonard began a relationship with Raj's sister Priya in season four, and it is implied they had a brief sexual encounter before the start of the series. The two attempted a long-distance relationship after Priya moved back to India, but Leonard eventually broke up with her in season five after she admitted to cheating on him with her former boyfriend. In season five's "The Recombination Hypothesis", Leonard and Penny start dating anew, and although their relationship is initially rocky and Penny has doubts, in season six, she works through her doubts and has a stronger relationship with Leonard.
In season six's "The 43 Peculiarity", she tells him she loves him for the first time, and this now makes them more comfortable in their relationship. Over the next two years, they talk about marriage and each have unsuccessful proposals until the penultimate episode of season seven, when they become engaged, and finally get married in the ninth-season premiere, followed with a wedding redo for their friends and families since the couple had eloped. In season, after Sheldon moves in with Amy, Leonard and Penny have apartment 4A to themselves. Leonard's family includes other accomplished scientists: his mother, Dr. Beverly Hofstadter, is a neuroscientist and world-renowned psychiatrist, his sister is a medical researcher, and his father, Dr. Alfred Hofstadter, is an anthropologist. Also, his brother Michael is a Harvard law professor. He is known to be lactose intolerant, and is also known for playing the cello.
Sheldon Lee Cooper (Jim Parsons) is a theoretical physicist, possessing a BS, MS, PhD, a Sc.D., and an IQ of 187. Born in East Texas, he was a child prodigy, as he had received his PhD at age 16. He is usually cold, self-centered, condescending, and immature, but at the same time it is commonly not intentionally hurtful and is in a well-meaning way; he just does not know any better. He does have a softer side, as he does care about his friends, and loves his mom, grandma, and girlfriend Amy, though he often insults them either about their intellect levels or different opinions. He shares an apartment in Pasadena with friend and fellow physicist Dr. Leonard Hofstadter.
Sheldon has a deep love and interest for science fiction and comic books, to the point of an obsession, and shows particular affinity for Spock, from the Star Trek franchise. He usually wears tee shirts emblazoned with logos for these interests as well as science-related designs. Sheldon refrains from any form of physical contact if possible and exhibits a strict adherence to routines to the point of exhibiting signs of obsessive–compulsive personality disorder. Amy confirms his disorder in the ninth season. Any disruption to routines distresses him enormously. A running gag has him always knocking on doors in a pattern of three knocks and one call repeated three times, which he has the urge to complete no matter what.
Sheldon has major difficulties in social settings; he shows great difficulty in identifying sarcasm and irony, and a recurring topic is his efforts to acquire those skills. His commitment to his friends is often called into question, as he is known for constantly and consistently being condescending and rude, many times even without being intentionally hurtful. He also has difficulties lying or keeping a secret, giving himself away with wild facial tics and implausible statements. He is, however, fond of occasionally telling a joke or playing a prank, which he typically punctuates with his trademark exclamation "Bazinga!". He has forced Leonard to sign an absurdly exhaustive "Roommate Agreement", and later enters a similar "Relationship Agreement" with Amy Farrah Fowler. Sheldon has a superiority complex and makes no effort to hide his contempt for other people's lower intellects, often making highly inappropriate comments. A common joke is his inability to drive, relying on those around him to help him get anywhere.
Unlike Leonard and Raj, who come from intellectual families, Sheldon stands out from his own relatives. His father, George, whose death antedates the series, is described as a stereotypical Texan who liked football, skeet shooting, and heavy drinking; his mother, Mary, is a devout Evangelical Christian, who does not seem to understand her son's fields of study, although she does display common sense and intelligence; his fraternal twin sister, Missy, and brother, George Jr., were both described by Mary as being "dumb as soup". His sister later has a baby boy, making Sheldon an uncle.
He meets Amy Farrah Fowler in the third-season finale and starts a highly unusual and slowly evolving relationship with her. Prior to this, he had exhibited some affinities with Leonard's equally unemotional mother. In the eighth-season finale, Amy takes a break from Sheldon, although the two resume their relationship halfway through the ninth season; as a sign of his love towards her, Sheldon decides to have sexual intercourse with her for her birthday in "The Opening Night Excitation". In the tenth season, Sheldon and Amy move-in together for a five-week experiment to test their compatibility and later decide to make the arrangement permanent. At the end of that same season, admirer Ramona Nowitzki make a romantic overture to Sheldon prompting him to propose to Amy.
Penny Hofstadter (maiden name has not been revealed) (Kaley Cuoco), is Leonard and Sheldon's neighbor across the hallway. Originally from a small town outside Omaha, Nebraska, she was a waitress and occasional bartender at the local Cheesecake Factory until season seven, and is an aspiring actress. According to her friend and neighbor Sheldon, she is a "good-natured simpleton". She is kind-hearted, outgoing, and does display the most common sense out of all the original five main characters. She also appears to be the most street-smart. Her lack of educational qualifications (she dropped out of community college) are a constant source of disparaging comments from Sheldon.
She has dated several men during the course of the series, including Kurt, Stuart, and Zack, but her recurring romantic interest is Leonard. Leonard pursued Penny during season one; they dated briefly at the start of season two, which led to some awkwardness; they began dating again and were a couple for most of season three, but she ended the relationship after he told her he loved her and she was not ready to reciprocate. Penny and Leonard begin to date again "slowly" during the fifth season. After several unsuccessful proposals from both sides, Penny realizes that her happiness is derived from her relationship with Leonard and they become engaged and later get married, though they upset their families by eloping, so they later have a wedding redo. She has a tense relationship with Sheldon, whose nerdy and obstinate personality often exasperates her, but they are ultimately good friends, helping each other in various situations. By season four, Penny also begins to socialize more with Bernadette and Amy, who frequently visit Penny's apartment, go out together, and comfort each other. Bernadette and Amy are both simultaneously envious of and threatened by Penny's more outgoing personality.
At the start of season eight, Penny pursues a career in pharmaceuticals and interviews for a job at Bernadette's workplace. She gets the job, and gradually becomes more independent, including earning a higher salary than Leonard. She reveals in season nine that she is not happy with her job, but insists on keeping it to help her eliminate her former credit card debts and secure a good financial future. In season 10, Leonard and Penny are finally living alone in apartment 4A after Sheldon moves in with Amy.
Initially, not much is known about Penny's family, but it is mentioned in the series that her father, Wyatt (portrayed by Keith Carradine,) raised her like a boy, her mother smoked marijuana while she was pregnant with her, her sister shot her husband while they were intoxicated, and her brother is a meth dealer. Her mother, Susan (Katey Segal), and brother, Randall (Jack McBrayer), are finally seen in season ten. [clarification needed], Penny bumped into her former boyfriend Zach and showed her ability to talk about topics related to Science and pronounce scientific terms perfectly.[episode needed] Also Penny has been fond of Yoga, which she treats as her way of relaxation and part of preparation for important events.
Howard Joel Wolowitz, M.Eng. (Simon Helberg) is an aerospace engineer at Caltech's Department of Applied Physics, who often hangs out at Leonard and Sheldon's apartment. Unlike Sheldon, Leonard, and Raj, Howard has no doctorate, and often gets disparaged as a result, especially by Sheldon. He defends this by pointing out that he has a master's degree in Engineering from MIT and that the equipment he designs is launched into space, unlike the theoretical work of his friends. In the fifth-season finale, Howard goes to the International Space Station on Expedition 31.
Howard lives in Altadena with his domineering, belittling, and unseen mother, who treats him like a child. While he sometimes expresses irritation at this treatment, for the most part, he appears to prefer it. Howard dotes on his mother and participates in many of her daily routines. A recurring gimmick in the series is Howard and his mother communicating with each other from different rooms by yelling, a habit which Bernadette also adopts in later episodes. His father left them when he was 11, and Howard gets visibly distressed when the subject comes up. Howard fancies himself a ladies' man and attempts pick-up lines whenever a woman is present, which often sound inappropriate and disgusting, although he has dropped this habit since he started going out with Bernadette. He is Jewish, but is not serious about his faith, and does not keep kosher. However, he does observe the Sabbath and the Jewish holidays.
In the pilot episode, he speaks English, French, Mandarin, Russian, Arabic, and Persian, along with fictional (constructed or colangs)  languages like Klingon and Sindarin. Howard suffers from asthma, transient idiopathic arrhythmia, allergies to peanuts, almonds, and walnuts, and is prone to canker sores and pink eye. Howard dated Bernadette briefly in season three. After some difficulties, they resumed their relationship in season four, which culminated in their engagement. In the fifth-season finale, Howard and Bernadette are married on the roof of the apartment building shortly before he has to leave for Kazakhstan, where he is to be sent to the International Space Station. In season six, he finally leaves his mother's house and moves into Bernadette's apartment. In the episode "The Comic Book Store Regeneration", Howard's mother passes away in her sleep while in Florida, prompting him to move back into his home with Bernadette. Stuart, the owner of the guys' favorite comic book store, joins them, much to their annoyance.
Upon learning that Bernadette is pregnant in season nine, Howard panics about handling the impending baby, particularly earning enough money. With the help of his friends, he designs a new form of guidance system that attracts the attention of the Air Force. In "The Birthday Synchronicity", Bernadette gives birth to a girl named Halley, on Amy's birthday, who screams like his late mother, Debbie Wolowitz.
In season 11, Howard learns that Bernadette is pregnant again. Which leads him to gain more maturity and he deals with life's challenges better.
Rajesh Ramayan Koothrappali, PhD (Kunal Nayyar) is Howard Wolowitz's best friend and another genius of the group; his name is usually shortened to "Raj". He is from New Delhi, India, and works in the physics department at Caltech, where his area of expertise is astroparticle physics. He lives in an apartment in Pasadena. As with his friends, he is mutually involved with and obsessed with science fiction and comic books in general. He is also a fan of Harry Potter.
Raj dislikes India, Indian food and Indian music, but appreciates the Indian lullabies his mother sang and the catchiness of Hindi phrases. Raj is a Hindu and believes in karma (reincarnation), but eats beef. He is very shy around women outside of his family, and during the first six seasons of the show, found himself unable to speak to women while in their presence unless he drank alcoholic beverages, or believed he had done so. When Penny, Amy, and Bernadette were around, Raj usually whispered what he wanted to say to Howard or Leonard, who then repeated or responded to what Raj said aloud. Despite this, Raj has sometimes ended up in bed with women, leaving the other characters perplexed.
Unlike his friends, Raj has many feminine interests, such as reading Archie comics and Twilight and watching chick flicks such as Bridget Jones's Diary. A recurring joke in the series has people, including his parents and Leonard's mother, speculating that Raj might be gay due to his feminine interests and his close friendship with Howard, with whom he has arguments similar to those of a married couple. However, Raj has always stated that he is straight, but metrosexual. He has had an infatuation with Penny, and secretly wrote love poems about Bernadette. Raj comes from a very wealthy family in India, and often communicates with his parents, Dr. and Mrs. Koothrappali, via webcam. He has a younger sister, Priya.
In season six, Raj started seeing a shy and sweet woman named Lucy, who had many things in common with Raj. However, she feels too pressured when he asks her to meet his friends and breaks-up with him. Afterwards, Raj finally speaks to Penny without having consumed alcohol. As of season seven, he no longer requires alcohol, drugs, or medication to speak in front of women. In season seven, Raj starts dating Emily Sweeney, a dermatologist, and their relationship becomes exclusive at the end of the seventh season. In the ninth season, he meets a bartender/scriptwriter and dates her along with Emily. However, in season ten, he mentions he is currently single. After his parents divorced, Raj's father ended his financial support and he was forced to move into Leonard and Penny's apartment. Due to friction between them, Raj moved into an apartment over Bert's garage.
After starting to live on his own and learning to manage his own finances, Raj has finally started to work at the Observatory where his expertise is more highlighted. This also paved the way for Raj to gain more confidence and become more mature. He has learned to demand more respect from Howard.
Additional main characters
Leslie Winkle, PhD (Sara Gilbert) is an experimental physicist who works in the same lab as Leonard. In appearance, she is essentially Leonard's female counterpart, equipped with the black-framed glasses and sweat jackets. She also happens to have a lazy eye. Leslie had casual sexual relationships with both Leonard and Howard, considering it a completely physical reaction to her body's cycles. She is one of Sheldon's arch-enemies, due to their conflicting scientific theories. Though they both consider each other to be intellectually inferior, Leslie is much wittier than Sheldon, regularly calling him "dumbass" and besting him in their exchanges. However, Sheldon did gain a small victory over her when he caused her and Leonard's break up after deliberately instigating an argument between the two.
Sara Gilbert made her first appearance in the third episode of season one and her final appearance as a regular in the third-season finale; Entertainment Weekly reported in January 2009 that she had been downgraded from starring status to recurring because the producers of the series were unable to come up with new storylines for the character. She returned for a guest role in the 200th episode, "The Celebration Experimentation", in season nine, with her character talking briefly about how she and Sheldon have changed over the years and affectionately calls him "dumbass" again at Sheldon's insistence (due to his dislike for changes).
Bernadette Maryann Rostenkowski-Wolowitz, PhD (Melissa Rauch) is originally a waitress and co-worker of Penny's at The Cheesecake Factory, using her wages to pay for her graduate studies in microbiology. She is a smart, short-tempered, ruthless, and often competitive young woman who sometimes displays a sweet, nice, and friendly personality. Despite her squeaky voice, she has a darker side. A recurring joke in the series is in reference to Bernadette and her lab team handling dangerous or infectious specimens, leading to accidental byproducts or exposure to diseases, in some cases resulting in Bernadette being quarantined.
Bernadette is of Polish origin and Catholic upbringing, and was originally seen wearing a cross necklace. She frequently manipulates people, often pretending to be a friendly and good-natured individual, quickly making friends with Penny and Amy, as well as the rest of the gang. Despite her short stature, she is regarded as being somewhat intimidating, largely because of her sharp tongue, aggressive demeanor, and occasional signs of mania. In season eight's "The Locomotion Interruption", Penny interviews for a pharmaceutical sales representative position with Bernadette's boss, and secures because she concurs with his opinion that Bernadette is kind of a bully and that they are slightly terrified of her.
Bernadette has demonstrated that she has some influence over both her father and her husband, including directly supervising Howard's expenditures by giving him allowance money. On one occasion, she was also able to influence Sheldon into a decision of which he originally disapproved.
Penny first introduced Bernadette to Howard on Leonard's half-willing request. During their first date, Bernadette and Howard do not find any common field of interest until they discover that they both have domineering mothers. After their third date, Bernadette declares that three dates is the threshold for deciding whether they want a long-term relationship. Blindsided, Howard hesitates initially, causing a temporary rift between them, but then impulsively proposes marriage to show her that he does care about her. Frustrated, she declines, but when he sings a heartfelt song for her at the Cheesecake Factory, she declares that it is the most romantic thing anyone has ever done for her and their relationship continues. A lengthy offscreen breakup occurs when she catches Howard engaging in cybersex, but Howard apologizes and she consents to resuming their relationship.
In the fourth season, she obtains her doctorate and is subsequently hired by a pharmaceutical company that offers her a high salary. While preparing for their wedding, Bernadette is shocked and repulsed when she learns about Howard's past sexual escapades, although once again, an apology and reconciliation occurs in the same episode. They marry the day before Howard leaves for his upcoming mission aboard the International Space Station.
During seasons six and seven, the newlyweds deal with Howard's difficulty in readjusting to life on Earth, the struggle to keep a healthy relationship with Howard's mother, and arguments over money. In the ninth-season episode, "The Valentino Submergence", Bernadette reveals that she is pregnant, informing Howard in the next episode. She comically reveals that the baby was conceived in Sheldon's bed, much to Sheldon's chagrin. Raj learns about the baby's gender and inadvertently reveals that the child is a girl. In "The Birthday Synchronicity" Bernadette gives birth to a girl named Halley who screams like her late mother-in-law, Debbie Wolowitz.
In season 11, Bernadette learns that she is again pregnant, while Howard is telling her Sheldon and Amy got engaged. Her doctor puts her on six-months bed rest, which makes her fond of watching detective and criminal-themed TV shows.
Amy Farrah Fowler, PhD (Mayim Bialik PhD) is a neuroscientist and Sheldon's love interest in the series. She has a PhD in neurobiology (Bialik herself has a PhD in neuroscience), with a research focus on addiction in primates and invertebrates, occasionally mentioning such experiments as getting a capuchin monkey addicted to cigarettes or starfish addicted to cocaine.
Raj and Howard found Amy through an online dating site after secretly establishing an account using Sheldon's name and information. The site matches her to Sheldon, and the two share many similar traits. Once she and Sheldon meet, she becomes, as Sheldon puts it, a girl who is his friend, but not his "girlfriend". The two initially communicate by computer, but at the encouragement of the group, she starts socializing with the gang in person. Interestingly, earlier in the series, in the physics bowl episode, Leonard's team mentions getting the girl from the Blossom TV series on their team as she earned a Ph.D in real life. This was Mayim Bialik who later played Amy Farrah Fowler.
Early on, Amy is essentially a female counterpart to Sheldon. She is coldly rational, displays little emotion, and is awkward in social situations ranging from endearing to embarrassing. In season five, after Amy goes out on a date with comic-book store owner Stuart, Sheldon decides to solidify their relationship as boyfriend/girlfriend with a 31-page "Relationship Agreement". Later in the season, Amy begins a campaign to increase Sheldon's feelings for her by becoming more involved in his interests, including video games and Star Trek, and treating him as his mother did. Sheldon says during Howard and Bernadette's wedding ceremony that he hopes the two of them are as happy together as he is alone, hurting Amy's feelings, but he spontaneously takes her hand for comfort later as Howard is launched into space.
By the beginning of season six, they are regularly holding hands at Amy's insistence, but Sheldon repeatedly resists all of her attempts to engage in further physical intimacy. By mid-season, Sheldon gets some intimate views of Amy when he cares for her while she has the flu, pushing him further toward a traditional relationship with her. She also provides Sheldon with consoling hugs, and receives a slap on the rear by an intoxicated Sheldon who tells everyone that she is a great gal.
Amy tries to move into Sheldon's apartment when Leonard temporarily vacates it, but is unsuccessful in her attempt. Sheldon, after being prodded by Barry Kripke about his girlfriend, lies that he is having sex with Amy, though he admits to Penny that he and Amy may actually engage in sex. Sheldon later makes the same admission to Amy, while engaging in Dungeons and Dragons intercourse game play. Nevertheless, her frustration is frequently played for laughs from that point onward. In the season-seven episode "The Locomotive Manipulation", Sheldon sarcastically says he will give Amy the romance she wants, then passionately kisses her on the lips for the first time and enjoys it. In "The Prom Equivalency", Sheldon and Amy admit to being in love with each other.
Early in their relationships, Amy was ambivalent and often condescending towards Penny and Bernadette, she later spends more time with them, even at the expense of her time with Sheldon. As her appearances have progressed, she has shed her strident, aloof personality for a stereotypically feminine and social one, although she retains some social awkwardness. Her feelings for Sheldon have also grown considerably over time, and she considers him to be ideal in almost every way, though she sometimes becomes as annoyed and frustrated by Sheldon's quirks as his other friends do.
Amy also intensely wants her relationship with Sheldon to progress to include sexual intimacy, a drastic departure from her early days as a member of the circle of friends. She has come to consider Penny a very close friend, sometimes referring to her as her "bestie", occasionally admitting her entire social life revolves around Penny. Penny is fond of Amy, but to her dismay, Amy often fails to grasp the concept of "girl talk", by talking about feminine hygiene and anatomy when in her presence and at times being brutally honest. Amy also often demonstrates an infatuation towards Penny, occasionally bordering on physical attraction, and she has demonstrated a somewhat condescending attitude towards Bernadette, though still considers her a close friend.
Amy can play the harp. In "The Scavenger Vortex", she mentioned that she attended Harvard University. In the final episode of season eight, "The Commitment Determination", Amy decides to take a break from her relationship with Sheldon due to the lack of physical affection. Sheldon had intended to propose to Amy, but she took a break from the relationship before he could ask her. Amy ultimately breaks-up with Sheldon after a serious disagreement, but they slowly rekindle their relationship and eventually have sex as a birthday present from Sheldon.
When Sheldon's grandmother visits in season nine, she confronts Amy for hurting Sheldon and unintentionally reveals the secret engagement ring in his possession. Although Amy was initially shocked, she became really happy to know that Sheldon loves her enough to have considered taking that step forward in their relationship. Amy later prepares a surprise birthday party for Sheldon, which he has not previously celebrated with the group.
After a plumbing accident makes her apartment uninhabitable in season 10, Sheldon and Amy embark on a five-week experiment living together to test their compatibility. Amy later admits she lied about how long the repairs were taking so that she could continue living with Sheldon. They eventually move into Penny's old apartment together, while Penny moves in with Leonard. Later, they decide to make the arrangement permanent. Amy is later offered a three-month course at a science camp in New Jersey and leaves with Sheldon's encouragement. She becomes angry when she learns of Sheldon spending time with Ramona Nowitzki, whom she suspects has romantic feelings for Sheldon (which she does). In the season finale, after Ramona kisses Sheldon, he visits Amy and proposes to her. The episode ends on a cliffhanger awaiting her answer. In the Season 11 premiere episode "The Proposal Proposal", which picks-up immediately after Sheldon's proposal, Amy's answer is further delayed by a phone call from Leonard and Penny, after which she finally says yes. She and Sheldon marry in the season 11 finale.
Stuart David Bloom (Kevin Sussman) runs the Comic Center in Pasadena, the comic book store that the men visit. Stuart is characterized by his low self-esteem and loneliness, which often result in pathetic attempts to engage with women and win favor with the main cast. This is despite owning his own (albeit troubled) business, and being a talented portraiture artist who attended the Rhode Island School of Design.
During Stuart's first appearances in season 2, he shows enough social skills to ask Penny on a date on two occasions, though neither ends successfully. In "The Hofstadter Isotope", his first date with Penny goes well for most of the evening, but when they return to her apartment, their run-in with Sheldon devolves into an endless argument between Stuart and Sheldon over the minutiae of Superman comics that puts Penny to sleep. In "The Classified Materials Turbulence", their second date unravels when in the throes of a romantic embrace, Penny calls him "Leonard", ending any future prospects for them as a couple. In season five, Stuart goes on a date with Amy, but Sheldon interrupts the date to solidify his relationship with her.
Stuart was the partner of Wil Wheaton in a trading-card game tournament in which they defeated the team of Raj and Sheldon. As proprietor of a comic-book store, Stuart has an almost encyclopedic knowledge of comic books and superheroes. In the season-four episode "The Toast Derivation", he mentions that he is in financial trouble and that the store is now also his home, and over the years, he has questioned the profitability or success running the store.
During "The Decoupling Fluctuation", Stuart becomes close friends with Raj and joins the gang as Howard's replacement while he is in space. When Howard returns, Sheldon wants to dismiss Stuart from the group, but Raj objects. Leonard tie-breaks on the basis that they are receiving a 20% discount in Stuart's store, and Sheldon relents after Stuart offers to go as high as 30%, thereby becoming a bigger and more prominent part of the core group. Sheldon has tried to hide his clear (and factually based) belief that Stuart is more of a hanger-on to the group of friends than a core member, most notably in a story where Stuart was part of a motley "test brunch" along with Burt the geologist and a neighbor who only spoke English she'd heard on TV, so Sheldon could work-out any hosting issues before trying an event with his "real" friends. Stuart actually got angry and told Sheldon it was clear they didn't even see him as a friend, leading Sheldon to sincerely apologize and try harder to be nice to Stuart for a period. Stuart is also limited vis-à-vis the group because he is a lot like Raj, a mostly-single guy around three couples, but Raj is best friends with Howard and closer friends with the others than Stuart.
After his comic-book store burns, Howard offers him a place to stay and a job as his mother, Debbie's, nurse. Stuart likes the job and develops a surprising closeness with Debbie, even staying long after she recovers, to Howard's annoyance. After Debbie's death, Howard and Bernadette move back into the house, with Stuart still staying. In the eighth-season finale, the two attempt to ask Stuart to move out, but are unable to bring themselves to do it after Stuart reveals it is his birthday. He eventually makes his own decision to move in the middle of season 9; however in the middle of season 10, he reveals that he has been evicted from his apartment and tries to use his helping with Howard and Bernadette's baby to move back in with them, much to Raj's chagrin. He does not always appear with the main group, as he was somehow not at Howard and Bernadette's during the birth of their 2nd child Neil.
Kevin Sussman was a recurring guest actor from seasons two to five, and was promoted to starring role for the sixth season onwards.
Stuart, now a part of the regular characters of the show, has offered to live with Howard and Bernadette in season 11, where in he helps in taking care of household chores to lighten Bernadette's daily things to do.
Emily Sweeney, M.D., (Laura Spencer) is a Dermatology resident at Huntington Hospital, whom Raj finds on an online dating site. Emily went to Harvard and is shown to have a slightly alarming personality trait; she delights in the macabre.
In season seven, after finding Emily's online dating profile, Raj has Amy contact her. Amy and Emily exchange emails and find they have a lot in common. Emily does not like Raj's inability to correspond with her directly; but, when he does agree to meet her, he acts weirdly and she leaves. Raj later bumps into Emily and apologizes for their previous encounter. She gives him another chance and agrees to go out to dinner with him. In "The Relationship Diremption", Raj and Emily have dinner with Howard and Bernadette. Upon meeting Emily, Howard recognizes her from a blind date they had sometime before he met Bernadette. The blind date ended badly, causing him embarrassment at the dinner, but no issues.
In "The Gorilla Dissolution", Raj and Sheldon bump into Emily at the movies with another guy. Later, she visits him to talk and reveals that the guy was her tattoo artist who had been asking her out for weeks and she did so just to end the obligation. Emily then spends the night; and, in the next episode, Raj reveals to the gang that Emily and he are exclusive.
In season eight, Raj introduces Emily to Leonard and Penny; however, Penny senses that Emily does not like her. Emily reveals that she is uncomfortable with the fact that Raj and Penny had a previously sexual encounter. Emily and Penny later sit down to settle their differences, with mixed results. Emily continues to socialize with the gang in season 8, and in the finale, she suggests to Raj that they have sex in a graveyard. Raj wonders whether he should break up with her because of her alarming personality. The next season, Raj somewhat rashly breaks up with Emily after meeting another girl, Claire, and fantasizing about starting a family with her, though Claire makes amends with her boyfriend and berates Raj for his actions. He tries to reconcile with Emily, but she shuts the door in his face. Later in the season, Emily and Raj reconcile after a tearful phone call. However, in season 10, Raj mentions that he is currently single. He and Howard invite Emily, Claire, Lucy, and Emily the deaf woman to Raj's apartment for an intervention to find out why women keep dumping Raj. Emily mentions that, although she enjoyed her sexual relationship with Raj, she was put off by his fear of his parents.
Laura Spencer was a recurring guest actress on the series in seasons seven and eight. During production of season 9, she was promoted to a "fractional" starring role; the upgraded status ensured her availability on an as-needed basis, without requiring her in every episode.
The following list is sorted in alphabetic order of first names.
Alex Jensen (Margo Harshman) is an attractive Caltech graduate student who appears in season six. Sheldon hires her to review his kindergarten and elementary-school notebooks for any possible Nobel Prize-winning research and attend to the tasks that Sheldon perceives a waste of his own valuable time. Amy is at first jealous until she sees Alex instead flirting with an unresponsive Leonard.
After repeated attempts by Alex, Leonard finally realizes that Alex is making advances toward him. While proclaiming his loyalty to Penny, Leonard is flattered. Sheldon interferes (with the perceived intention of mitigating a situation that needed none) by lecturing Alex, but only succeeds in insulting her. After Alex files a sexual harassment complaint, Sheldon is forced to apologize and is ordered take a university-mandated online sexual-harassment course. Because Sheldon considers his own time as too valuable to waste on it, Sheldon asks Alex to take the course instead. For Valentine's Day, Sheldon sends Alex out to buy a present for Amy, which he likes so much, he keeps.
Arthur Jeffries, or Professor Proton (Bob Newhart) is the star of a science show that Sheldon and Leonard watched as children. After the show was cancelled, Jeffries was not taken seriously as a scientist and resorted to doing children's parties as his persona. In "The Proton Resurgence", Sheldon hires him to do a private party for Leonard and himself (and Penny). Later, Jeffries has a heart attack as a result of climbing the apartment stairs with his equipment. He asks Sheldon to take his place at a party for a Korean family, which Sheldon agrees to do as "Professor Proton, Jr".
While Sheldon idolizes him, Jeffries is constantly irritated by Sheldon's personality, and takes a shine to Penny. In "The Proton Displacement", Leonard, Sheldon, and Amy bump into Jeffries at a drugstore, and Sheldon becomes upset when Jeffries asks Leonard for help on a paper instead of him, and when he finds out that Jeffries thinks he is annoying, Sheldon works with Bill Nye the Science Guy, whom Jeffries accuses of stealing material from his TV show. After Nye abandons Sheldon and files for a restraining order against him, Jeffries arrives at Sheldon and Leonard's apartment and asks Sheldon for his advice on his paper (although Sheldon already hacked his email account and read it for himself).
In "The Proton Transmogrification" on Star Wars Day, Leonard tells Sheldon that Jeffries has died. Sheldon immediately pretends like he does not care and refuses to go to the funeral. Jeffries appears to Sheldon in a dream, where Sheldon compares him to Obi-Wan Kenobi, dressing him in Jedi robes and making him appear as a Force ghost. When Sheldon asks for his wisdom, Jeffries tells Sheldon to appreciate everything in life, including his friends, and never take it for granted.
Jeffries returns in "The Opening Night Excitation" when his ghost convinces Sheldon to spend time with Amy on her birthday instead of going to see Star Wars: The Force Awakens as he had planned to do for months. After hearing Professor Proton's words, Sheldon decides to prove his devotion to Amy by finally having sex with her, as she has been hoping he will do for the past few years. Some time later, after Sheldon takes Amy to see The Force Awakens, Jeffries returns to Sheldon to ask him about his first sexual experience with her, but Sheldon thinks Jeffries is talking about the film and tells him he enjoyed it.
Barry Kripke, PhD (John Ross Bowie) is a co-worker of the guys, who frequently clashes with Sheldon. He works in plasma physics. Kripke has a case of rhotacism in which he pronounces the letters "R" and "L" as "W" in much the same way as Elmer Fudd from Looney Tunes and Gilda Radner in her "Baba Wawa" sketches. In season five, Kripke purchases an iPhone with voice-recognition technology, but, because of his rhotacism, the device is incapable of understanding his verbal "wequests", prompting him to deride it as ineffective. Until season seven, whether he was aware of his impediment was never clarified.
In his first appearance, he pits his robot, the Kripke Krippler, against the men's robot, M.O.N.T.E., in an unofficial robot fight. The guys lose to Kripke, and M.O.N.T.E. is destroyed. In "The Friendship Algorithm", Sheldon attempts to befriend Kripke to gain access to an open science grid computer to conduct research, but it turns out to be futile, as Kripke has no control over the computer's usage time, and Sheldon consequently expels Kripke from the group. Kripke continues his antagonism towards Sheldon in "The Vengeance Formulation" by pulling a prank on Sheldon when he is a guest on NPR's Science Friday by putting a helium tank in his office. In "The Toast Derivation", Sheldon tries to befriend Kripke and a few others when he is feeling excluded by his friends, but Kripke and the others all connect in a manner that Sheldon finds 'wrong' and leaves.
In season five, Kripke and Sheldon battle over a retired professor's office; but, after a long sports contest, Kripke loses to Sheldon. He is later invited to Howard's bachelor party, where he uses his turn toasting to complain that no strippers are at the party.
In season six, Kripke and Sheldon are forced to work together on a proposal involving fusion reactors, which involves an exchange of work. Sheldon is shocked to discover that Kripke's work is more advanced than his own, and he tearfully admits as much to Amy. Kripke assumes that Sheldon is in a sexual relationship with Amy and that has led to his work quality declining, and Sheldon does not deny it (although this appears to be just a convenient excuse since his work is inferior to Kripke's). Kripke also battles for tenure alongside Sheldon, Leonard, and Raj.
In season seven, Kripke ridicules Sheldon when his discovery of a new element was false, nicknaming Sheldon "The Retractor".
In season nine, Kripke calls Sheldon and Amy's live show Fun with Flags along with Raj and directs the conversation to irrelevant questions to the show. He also uses the show to call out for potential romantic relationships and asked Amy questions about her love life when Sheldon and she broke up. He attends Sheldon's birthday party and constantly hits on Leonard's mother, Beverly.
Professor Bertram Kibbler (Brian Posehn) is a geologist at Caltech, who met Amy while she was working there in her own lab in "The Occupation Recalibration". Bert brings Amy interesting and beautiful rocks every day. It takes Howard and Raj to point out that he likes her. Bert then asks her to a rock and mineral show, which she declines because she has a boyfriend. When Bert starts talking about how no one wants to do anything with him, Amy in a fit of sympathy agrees to go. Unfortunately, Bert is taking it as a date. Raj and Howard offer to talk to him to let him down easy, and they end up going with him to the show.
Bert first appeared in "The Contractual Obligation Implementation" where he is socially awkward and Raj meets just prior to a library date with Lucy. He tried to crash their date, and Raj sent him away. His name was not mentioned in the plot, but the closing credits gave the name Bert.
In "The Dependence Transcendence", Bert hosts a Caltech party that only Amy and Penny attend. He feels that Amy is the coolest girl at work and that Sheldon is cool only because he is dating Amy. As the girls are leaving, he tells them that he is in love with both of them.
He reappeared in "The Fetal Kick Catalyst" as a guest at Sheldon and Amy's brunch in their apartment. He mentions that a girlfriend he was living with once moved out and took all his best rocks.
In "The Geology Elevation", Bert wins the McArthur Fellowship Grant from Caltech, which makes Sheldon jealous. When he is finally confronted by Sheldon over the latter's jealousy, Bert points out that the theoretical physicist has his friends and Amy to be happy about. He later accompanies Sheldon to The Ellen DeGeneres Show to make peace. His full name is shown in his award. By the end of season 10, Raj moves into the apartment over Bert's garage.
Bert reappeared in "The Separation Agitation", and it is revealed that he has a recent girlfriend named Rebecca, who only appears to be with him for his grant money. After breaking-up with her, Bert instantly regrets it and buys expensive gifts to successfully win her back. And at the episode, he is revealed to have won her back.
Beverly Hofstadter (Christine Baranski) is Leonard's narcissistic, unloving, and overly analytical mother. Beverly is a neuroscientist, as well as a psychiatrist. She is Sheldon's female equivalent regarding neurotically strict speech patterns, lack of respect for social conventions, and compulsive attention to detail. The only reason she had children was to use them as personal experiments for her research, so was very distant and unmotherly towards Leonard. Beverly mentions that Leonard's brother and sister are more successful in their respective fields than he is, though she respects this enough to keep contact with them (unlike Leonard); she is not proud, since they are not her accomplishments. One of her famous books is Needy Baby, Greedy Baby, in which she describes an infant's (Leonard's) unrealistic nurturing expectations. Sheldon often refers to her books when frustrated with Leonard's attempt to obtain some of his personal needs in their relationship. Leonard sees his mother as cold and not nurturing, even telling Penny that one time he made a hugging machine just to get hugs (and that his father asked to borrow it). The lack of Beverly's maternal feelings and actions toward Leonard has led to him being called the "King of Foreplay" because of his obsessive need to please.
In her first visit, Beverly makes Penny cry by delving into her history with her father (driving her, and later Leonard, to the bottle), and considers Raj and Howard's relationship to be an "ersatz homosexual marriage". After bonding with Sheldon, Beverly sings Journey's "Any Way You Want It" with him on karaoke.
In season three, Beverly comes to visit at Christmas, where she reveals that she is divorcing her husband, Alfred, after she caught him in bed with another woman. Despite Leonard's reluctance to tell Beverly about his and Penny's relationship, Penny takes her to the Cheesecake Factory for a round of drinks and reveals their relationship. After Beverly gropes a busboy, she and Penny both go to Del Taco and then return to Leonard and Sheldon's apartment where she confronts Leonard on not telling him about Penny, and then is confronted by him for not being a loving and caring mother for him. After Beverly tries to show motherly affection by hugging Leonard, she kisses a shocked Sheldon and leaves. The next morning, a hungover Beverly gives her blessing to Leonard and Penny's relationship and vows not to reveal her drunken kiss with Sheldon, wanting to work instead on her maternal relationship with Leonard.
In season five, Sheldon calls Beverly on Skype to help Leonard following Penny and Raj's hook-up, and she tells Leonard to "buck up, sissypants...If you need any more help from me, my books are available on Amazon".
In season seven, Leonard uses Beverly's latest book to get sympathy from Penny. However, Bernadette finds out from Howard what Leonard is doing and tells Penny, who calls Beverly, leaving a briefly pantsless Leonard to chat with his mother on Skype. Leonard later informs his mother of his engagement to Penny, but she reveals that Sheldon already told her. She decides to accept his engagement with Penny because Sheldon likes her.
In season eight, Beverly comes to visit to see Leonard receive an award, where she meets Sheldon's mother, Mary. The mothers get into an argument after Beverly describes Mary's religious beliefs as a superstition, and Mary, in turn, berates Beverly for her cold and distant attitude towards Leonard. Beverly later reflects with Sheldon that perhaps other ways exist to raise children than she did. The mothers make up and Beverly tries to show affection by hugging Leonard, though it is awkward for both of them.
In season nine, she attends Sheldon's birthday party and enjoys a proposition from Barry Kripke. Later, she returns for a visit to interview people for her new book, and spends her time bonding with Amy and Bernadette, frustrating Penny's attempts to bond with her. When Penny confronts her for this, she admits that she feels insulted about not being invited to her son's wedding. Penny suggests doing a second wedding ceremony for her, which she accepts and the two finally bond. She is unhappy about her ex-husband, Alfred, attending since their marriage ended due to his affair, and is even angrier when Alfred and Mary Cooper display mutual attraction and leave the rehearsal dinner together. Beverly and Alfred argue all night and Beverly once again quarrels with Mary. At the ceremony, she agrees with Alfred that Leonard was the best thing to come out of their relationship.
In season 11, Beverly begins talking to Penny as a friend and confidant, troubling Leonard, especially when he learns that Beverly told Penny that she is proud of her, a level of praise she never bestowed upon Leonard himself. When Leonard confronts Beverly, he is touched when she says that of all of her children's spouses, Penny is the one she is most impressed by, and that for this, she is indeed proud of him.
Claire (Alessandra Torresani) is a writer for a children's science fiction series whom Raj and Howard meet in the comic book store in season 9. Claire wants to collaborate with Raj on the science portions of a movie script. When Raj reveals to her that he has a girlfriend, Emily, she assures him that she is only interested in his technical assistance, and they can still meet as friends. However, Raj develops visions of marriage and children with Claire. After Raj breaks up with Emily, he calls Claire on Valentine's Day, only to learn that she has just got back together with her boyfriend. She also calls Raj an "ass" for breaking up with Emily just before Valentine's Day. Later, however, she asks Raj out for a drink after she has again broken up with her boyfriend. However, Raj is on his way to reunite with Emily, and chooses to be with her. However, Raj later starts seeing Claire again while at the same time dating Emily. She and Raj continue to date, but they keep their relationship casual with no commitment. When she meets Raj's friends, Claire begins to realize that Raj has been dating multiple women, and successfully tricks him into admitting it.
In season 10, Raj reveals that he is now single, having broken up with both Emily and Claire at the ladies' behest. Later in the season, when Raj tries to figure out what is wrong with himself, Claire accepts an invitation to Raj's apartment, where she meets his previous girlfriends. Claire reveals that she broke up with Raj after finding him too needy and vain and declares that she is now in a better relationship.
Dan (Stephen Root) is a senior member of staff at Zangen, who first appears in "The Locomotion Interruption" when he interviews Penny for a pharmaceutical sales representative position as a favor to Bernadette. Penny realizes that the interview is not going well so she ends it and starts to leave. As a favor, she asks Dan not to tell Bernadette how badly she did because she is kind of scared of her. Dan says that he thought he was the only one who was scared of her. They start to bond, agreeing that Bernadette is something of a bully. Dan comments that has that squeaky little voice, and that everyone else thinks she is SO nice. He had to cut one of Bernadette's research programs, but was too scared of her to do it. Dan and Penny both agree not to tell Bernadette about the results of the interview. Later Bernadette is talking to Howard at home, and tells him that it is hard to believe, but Penny got the job. She feels that it is because Dan really loves her.
Dan reappears in "The Champagne Reflection", when Penny makes Bernadette face up to her bullying ways. Dan is still afraid of Bernadette, and tells the story of how she yelled at him and his grandson after they lost the three-legged race at the company picnic. His grandson called her the mean kid with the big boobies. Bernadette is horrified when she realizes how everyone perceives her. Her father had taught her to be tough and she can be cruel with some of her comments without even knowing it. She says she thought she was the sweetest person she knew, and should be in a tree baking cookies like a Keebler elf. She goes to apologize to Dan and discovers that, owing to everyone's fear of her, they have been paying for her daily coffee even after the company stopped doing so (something they accomplished with a swear jar kept for Bernadette's sake); and have given her the private use of a toilet that was meant for everyone. She now feels like she has to leave the company and starts crying. Desperate for her to stop, Dan promises to keep paying for her coffee, and even offers to get her an espresso machine. Bernadette has found a new way to get what she wants.
Dave Gibbs (Stephen Merchant) is a tall British man who dates Amy after she breaks up with Sheldon. Dave is recently divorced because his wife left him for a French chef. When her friends try to find Amy a date on a dating mobile app, they see a text message from Dave, and Amy reveals they have already gone out together for coffee. Sheldon is about to propose to Amy for closure when he sees Amy and Dave share a kiss after their date. When Amy and Dave later go out again for dinner (while Penny, Bernadette, and a reluctant Leonard spy on them), he shows an unusual fascination with Sheldon, which discomforts Amy. He considers Sheldon and Leonard to be physics superstars and idolizes them. He later gets an opportunity to meet Leonard and shake his hand after Leonard has to relieve himself while spying on their date, and with great excitement, Dave announces that he will never again wash his hand. After Sheldon rejects Amy's proposal that they be a couple again, Amy once again tries dinner with Dave, who goes out of his way to refrain from mentioning Sheldon, knowing how uncomfortable that subject made her previously. Sheldon then suddenly arrives and professes his love to Amy, and the two mend their relationship, leaving Dave to awkwardly excuse himself and invite Sheldon to meet again on his way out.
Eric Gablehauser (Mark Harelik) is the head of the Physics Department, the group's boss at the university. When he first starts working at the university, he fires Sheldon after Sheldon insults his intelligence. Eventually, Gablehauser rehires Sheldon after he and Sheldon's mother display a mutual romantic interest. Gablehauser later hosts the Physics Bowl, and gives Raj attention when he is featured in a People article for discovering a planet.
V. M. Koothrappali
V. M. Koothrappali (Brian George) is Raj's father in India. His wife and he communicate with their son via Skype and constantly try to arrange dates for him. They want their son to marry a woman of Indian descent, and give them grandchildren. Although in many episodes Raj mentions that he grew up in poverty, his friends remind him of the contrary by pointing out that Dr. Koothrappali is a gynecologist, drives a Bentley, and employs servants. In one such episode, Raj replies that the Bentley is leased and there are only four servants in his family home, two of whom are children. Dr. Koothrappali enjoys Doogie Howser reruns, which are apparently new to India. Other than Raj, he also maintains contact with their daughter Priya via Skype whenever she is out of India. In season four, Dr. Koothrappali becomes angry with Leonard when he learns that his daughter, Priya, is dating Leonard without his knowledge. According to Sheldon, the Koothrappalis are "Richie Rich rich".
The Koothrappalis go through a rough patch in their marriage, culminating in them separating in season eight around the time of their 40th wedding anniversary, with Dr. Koothrappali moving out and Mrs. Koothrappali setting fire to his car. Dr. Koothrappali makes his first physical appearance on the show in season eight, visiting the main characters at a Christmas party after the divorce. Dr. Koothrappali later cuts off Raj financially when he buys a helicopter drone, but Raj manages to turn the situation around by pitting his father against his mother. He also asks his father to give advice to a nervous, pregnant Bernadette.
Mrs. Koothrappali (Alice Amter) is Raj's mother in India. Mrs. Koothrappali is especially worried that, despite Raj being old enough to marry, the closest they have to a daughter-in-law is "that little Jewish boy Howard". She enjoys Doogie Howser reruns, which are apparently new to India. Other than Raj, she also maintains contact with Priya via video chat whenever she is out of India. She also had some anger towards Leonard when it became apparent that he was dating Priya without her knowledge.
Following their divorce in season eight, Raj mentions that his mother bought the book Eat, Pray, Love and used it to set fire to her ex-husband's car. Mrs. Koothrappali makes her first physical appearance on the show in "The Graduation Transmission", where Raj uses the Koothrappali's divorce to get money from her when his father cuts him off financially, and pits her against her ex-husband.
Janine Davis (Regina King) is a human resources representative working at the university. She first appears in season six after Sheldon offends his assistant Alex while trying to solve a problem regarding her crush on Leonard, leaving Mrs. Davis to handle Alex's sexual-harassment complaint against Sheldon. Sheldon tells Mrs. Davis his belief that she, like all women, is a "slave" to her sexual "urges", unaware that his remarks are racist and sexist. In his attempt at defense, Sheldon rats out what he considers similar behavior by Raj, Howard, and Leonard, prompting Mrs. Davis to summon them to her office, as well. Mrs. Davis later serves on the tenure committee that is considering Raj, Sheldon, Leonard, and Kripke for a vacant tenured position, causing each of them to try to win her favor. Sheldon, in his effort to earn the position, offends her once again by giving her the box set for the miniseries Roots. Despite this, Sheldon is still short-listed for tenure, even if he offends Mrs. Davis yet again by giving her what he thinks is a traditional handshake originating in African American communities.
In season seven, Mrs. Davis says that she is recently divorced because her husband cheated on her, and Raj attempts to strike up a conversation with her at a university mixer. After offending her, Raj apologizes, and the two talk about their troubles. Following the mixer, while Raj is insistent that Mrs. Davis and he had a "moment", Howard disagrees. Mrs. Davis makes another appearance in season 8, where she informs Sheldon that he has to teach a class as a junior professor, and in season 11, has to deal with Leonard after he gives an embarrassing interview on the radio about the university.
Kurt (Brian Patrick Wade) is a tall, intimidating bodybuilder and Penny's ex-boyfriend at the beginning of the series. In the pilot, Penny reveals that he cheated on her, and Leonard and Sheldon later go to his apartment and attempt to retrieve Penny's television set from him, but they were unsuccessful and Kurt apparently found them so irritating that they returned home without pants. Kurt is invited to Penny's Halloween party, for which he dresses up as a caveman, where he has yet another run in with Leonard, who is dressed as a hobbit. After Kurt sees Penny kissing Leonard, Leonard victoriously shouts to him, "That's right, you saw what you saw. That's how we roll in The Shire!"
In season two, Penny experiences financial difficulties. After Leonard learns about Penny's problems, he tries to help her cut expenses, and in the process learns that Kurt owes her $1800. Without telling Penny, Leonard and the guys go to Kurt's place to collect the money, but they are unsuccessful, and Kurt writes "I owe Penny $1800 ~ Kurt" on Leonard's forehead in indelible ink. Later, Penny reveals that Kurt visited her the next morning and paid her, in turn. However, much to Leonard's disappointment, Kurt never told Penny about Leonard's visit, and instead took the opportunity to ask Penny out for dinner.
Lucy (Kate Micucci) shows up at a party at the comic-book store on Valentine's Day at an event for people who had no dates. She has social anxiety issues, which allows Raj and her to connect. On their first date, she becomes so nervous that she goes to the bathroom and climbs out of the window to get away. Raj falls apart and refuses to leave his apartment; however, after apologizing to him, Lucy and Raj start afresh. Raj and Lucy have a date at a library and communicate by text messages, and at the end, she offers to kiss him, but chickens out at the last second. In "The Bon Voyage Reaction", Raj tries to introduce Lucy to the rest of the gang. Lucy becomes nervous at the thought of meeting so many new people that Raj suggests she meet just one of his friends first, to which she agrees. Lucy meets Amy, but Raj pressures Lucy and upsets her. Lucy breaks up with Raj by an email, and afterwards a heartbroken Raj finally speaks to Penny without having consumed alcohol. Originally, Kate Micucci had been one of the actresses considered for the Amy Farrah Fowler character.
In season seven, Penny meets Lucy at the Cheesecake Factory and confronts her about her breakup with Raj, and later Lucy arranges to meet with Raj to apologize to him for breaking up with him via email. This leads Raj to believe the two of them will get back together, but Lucy tells him she is with someone else. Although she is not seen, Lucy later texts Raj saying that she misses him and wants to get back together, but after finding out that he is also dating Emily, Lucy rebuffs Raj.
Lucy appears in season 10, where Raj invites her to his apartment along with his other ex-girlfriends to find out why he is still single. Lucy mentions that she is now in a better relationship.
Mary (née Tucker) Cooper (Laurie Metcalf) is Sheldon's loving, caring and religious mother and a devout Christian from Texas. She has two other children besides Sheldon; Sheldon's twin sister, Missy, and his brother George, who is shown via a family tree to be three or four years older than Sheldon and Missy. Her husband, a rambunctious alcoholic, was also named George, but he died when Sheldon was still a boy. To Mary's relief, her other children do not share Sheldon's hyperintelligence, once commenting to Leonard, "I thank the good Lord my other kids are as dumb as soup". Mary herself is not intellectual, but is quite wise, though she is prone to making insensitive comments such as calling Japanese words "kung fu letters" at a sushi restaurant, or calling Native Americans ""our" Indians". Despite her extreme religious views, she is tolerant of other faiths, as evidenced by her serving Raj a dish of chicken after making sure it was not something Indians considered "magical". She also struggles to follow her beliefs sometimes. She is a loving mother and has been able to control Sheldon, with Leonard describing her as Sheldon's "Kryptonite" to Penny. When Penny and Sheldon engaged in a fierce dispute, Penny called for Mary's help, who subsequently called and scolded Sheldon about his actions. Leonard wishes his mother were as loving as Sheldon's mother, although Sheldon himself appears at best ambivalent about her parenting. Sheldon once mentions that Mary once hit him with a Bible because he refused to eat his Brussels sprouts. She apparently thought something was wrong with her son while he was growing up, as Sheldon will often remark after someone calls him crazy, "I'm not crazy; my mother had me tested!" Though Mary confirms this, she wishes that she had taken him to Houston for further testing.
In her first visit, Leonard calls Mary to intervene when Sheldon is fired for being disrespectful to his new boss, Dr. Gablehauser. She manages to convince Sheldon to apologize and he gets his job back, and she becomes romantically interested in Dr. Gablehauser. When Sheldon asks, "Is Dr. Gablehauser going to be my new daddy?", Mary replies, "We'll see".
In season three, after returning from the North Pole, Sheldon returns to his home in Texas after the other guys tampered with his research. Mary calls the guys and they come to fetch Sheldon. Sheldon initially refuses to leave, although after an argument with his mother about evolution, Sheldon decides to leave. Mary is again called to intervene in "The Zazzy Substitution" when Sheldon buys a clowder of cats after his "break-up" with Amy. Mary uses reverse psychology to get Sheldon to reconcile with Amy. She again comes to visit in "The Rhinitis Revelation" before going on a Christian cruise, and Sheldon becomes jealous when she would rather go sightseeing with the rest of the gang instead of cooking for him and attending physics lectures.
In season seven, Sheldon returns to Texas to assist Mary in the birth of his twin sister's baby. Howard later takes Sheldon to see Mary after visiting NASA headquarters, but flees after Sheldon sees her having sex. Sheldon returns and gets into an argument, and is sent to his room. After Howard tells him about how he drove away an admirer of his mother, Sheldon apologizes to Mary, saying that he will accept her choices on the outside while condemning her internally for being a hypocrite by going against her religious beliefs. Mary forgives him.
In season 8, Mary meets Dr. Beverly Hofstadter, Leonard's mother. The two very different women start out as civil, but later quickly come to clash as Beverly insults and belittles Mary strict religious beliefs, and Mary in turn derides Beverly's cold attitude. Mary is especially insulted when Beverly is left dumbstruck how a genius intellect like Sheldon could be born to a woman like her. After taking a break from each other, the two come to terms with the argument: Mary decides to forgive Beverly for her offhandish ways, and Beverly realizes there is more than one way to raise a child. The two mothers make up and Beverly tries unconditional love by trying an awkward hug with Leonard.
In season nine, Sheldon calls her to tell her that Amy broke up with him and that he would like to send the family heirloom engagement ring back, but she advises him to hold on to it, just in case. She reappears in the season finale "The Convergence-Convergence" where she comes to attend Leonard and Penny's re-wedding where she is not at all happy to see Beverly. She meets Leonard's father Alfred Hofstadter and the two get along great as they have each other's nice, gentle, and caring personalities, bonding over their mutual dislike of Beverly, and their admiration of each other's beliefs. The two of them end up fighting with Beverly at the restaurant during the pre-ceremony dinner and the two leave together and after finding out they are staying at the same hotel, they decide to share a cab and it is implied that the two hook up. In season 10, during a visit to see her, Sheldon and Amy break it to her they are living together and Sheldon is stunned that she is not offended that they are "living in sin" due to her belief he was never going to find anyone.
Mary has argued with Leonard's mother, Beverly, over the fact she never nurtured Leonard properly the way she has with Sheldon, while Beverly has criticized her for her narrow-minded beliefs.
Mike Rostenkowski (Casey Sander) is Bernadette's father and a tough retired police officer. Mike has a rough and rude personality and often bullies others, although he deeply loves his daughter, calling Bernadette his "little girl". Before he is first seen, Bernadette establishes his personality by telling Howard a long list of subjects he cannot discuss with Mike, including Jimmy Carter, foreigners, homosexuals, and even Howard's Jewish identity.
In his first appearance, he tells Howard, while he does not think highly of him, his respect for Howard grew when he learned he had been selected to become an astronaut. However, after Howard expresses his fears about not returning from space, Mike believes Bernadette will have no trouble finding another suitable bachelor if Howard dies in space, much to the future astronaut's dismay. In the season-five finale, Mike attends Howard and Bernadette's wedding and walks his daughter down the aisle.
In season six, Howard and Mike are pushed into a fishing weekend by their spouses, but neither man really wants to go. Mike suggests that they go to a casino to shoot craps. Having earned more of Mike's respect, Howard is now able to address him as "Mike" rather than "Sir".
In season seven, because his wife is out of town, Bernadette gets Mike to come to Howard's mother's house for Thanksgiving dinner, though he would rather watch football at home. Since Sheldon understands football from growing up with his father, Mike and Sheldon bond and drink heavily together. After insulting Howard, Mike later tells Howard that he is not "all that bad", but implies that this statement is untrue when he comments on his drunken state. In "The Table Polarization", Mike, Bernadette, Raj, and Mike Massimino (via Skype) perform an intervention on Howard's decision to go back to space. Mike mentions that Howard asked him to shoot him in the foot to avoid the first space mission, which Howard insists was a joke.
In season nine, he comes to Howard and Bernadette's house since they are talking about renovating and he comes over to help. While Howard and he are busy together, Mike asks Howard why he does not want kids, but Howard sets things straight when he tells Mike that it is actually Bernadette who does not want kids and she causes the dust avalanche on both of them. Bernadette had lied to her father about not wanting kids because she did not want to disappoint him. Mike says he does not want to see her miss out since raising children was the most rewarding experience of his life, but Bernadette points out that her mother did everything while all Mike did was come home from work, sit on the couch, and drink beer, which is unrelated to raising kids. Defeated, Mike leaves, saying this is really a conversation for husband and wife and tells Bernadette if her mother asks, he was at the Wolowitz house until 10. He briefly comes back into the kitchen and sees Howard and Bernadette doing "dirty talk" and then disappears again through the doorway.
President Siebert (Joshua Malina) is the president of the California Institute of Technology, is married and has a teenaged daughter. Siebert is often mentioned by Sheldon. A recurring gag is that Sheldon often calls or visits Siebert in the middle of the night to get information, and usually gets a bitter response. Siebert sends Sheldon on the expedition to the North Pole at the end of season two.
In his first appearance, Siebert organizes a get-together with wealthy donors and faculty members to garner funds for the university's programs. He urges the guys to make an appearance to schmooze the donors so they will give to their projects. Seeing Sheldon's behavior toward the donors, he asks that he not appear again.
In season five, Siebert forces Sheldon to use up his vacation days, and when Sheldon and Kripke battle for an office at the university, they interrupt Siebert at a urinal to get him to make a decision on who gets the office.
Priya Koothrappali (Aarti Mann) is Raj's younger sister. Having graduated at the top of her class at Cambridge University (where like Leonard, her roommate was from Texas), she is one of the lead attorneys at India's biggest car company.
In her first appearance, Priya has a two-day layover and comes to visit her brother. Leonard and she are revealed to have had a relationship five years earlier (without her parents' or Raj's knowledge) when she was in town, and that Leonard had been much more invested in the relationship than she was. Leonard even offered to relocate to New Delhi to be with her. He tries to keep their relationship a secret from the guys, but Sheldon discovers Priya in their apartment after spending the night together. Leonard eventually reveals to the rest of the guys that he spent the night with Priya, as he is unable to bear the burden of keeping the secret. Priya returns to Los Angeles for business in "The Cohabitation Formulation", and Leonard and she decide to try to have a real relationship, over the protests of Raj. It upsets Raj to know that his sister is having sex with one of his best friends (sometimes in Raj's bed). Priya soon becomes jealous of Penny and asks Leonard to stop spending time with her, which causes Penny, Amy, and Bernadette to dislike her. Priya contests Sheldon's roommate agreement with Leonard, employing her legal background to deny Sheldon power over him, but Sheldon drafts a new agreement and blackmails Leonard into signing it by threatening to tell Priya's parents she is dating Leonard, which she cannot bear because they want her to date an Indian (although her father would accept her dating Howard due to his belief that Jews are financially savvy and do not drink much). At the end of season four, it she is moving back to India and has not told Leonard, which Leonard interprets as the end of their relationship, leading him to expose their secret relationship to her parents.
In season five, although they are separated, with Priya moving back a month earlier than expected, they nonetheless remain in touch via Skype, having a cyber-dinner/breakfast date. At Howard's suggestion, Leonard and Priya also attempt to simulate intercourse, but when it comes time for her to remove her clothing, Leonard's screen freezes up because of his exceeding the bandwidth. Leonard later manages to get cue cards to help himself with a seduction speech to Priya, but while delivering it to her, he inadvertently reveals to her parents that they are cyber-dating (they were in the same room as she when he called), with them acidly asking for him to continue with the speech. In "The Good Guy Fluctiation", Leonard tells Priya about his attraction to a comic-book enthusiast named Alice; Priya admits to cheating on him with her former boyfriend, which visibly upsets Leonard. When Priya states that they both messed up a little, Leonard retorts that while he messed up a little, she messed up a lot. She claims that it is not a competition, to which he replies that it is and that she won and he switches off his laptop. In the next episode, Leonard refers to himself as single, indicating that Priya and he have broken up as a result of their last conversation. In season 10, it is mentioned that out of the six Koothrappali children, only Raj is still single, indicating that Priya has married since her last appearance.
Ramona Nowitzki, Ph.D., (Riki Lindhome) first appears in the Season 2 episode, "The Cooper–Nowitzki Theorem", when she is a graduate student at Caltech who is a huge fan of Sheldon's work and develops a near-obsessive crush on him. She meets Sheldon personally in the cafeteria and arranges to have dinner with him in his apartment. Where other people might be worried about such behavior being reminiscent of stalking, Sheldon reacts positively to Ramona by appreciating the fact that he is getting a free dinner. During their date, the rest of the gang is sitting on the couch as spectators of this strange turn of events. She keeps "helping" Sheldon by arranging suitable working conditions for him, such as getting breakfast for him at the cafeteria and giving him foot rubs. Unfortunately for Sheldon, she overdoes her insistence that Sheldon dedicate his time solely for work, depriving him of his favorite hobbies such as playing games, watching TV, and going for paintball with the guys. Even more unfortunately for Sheldon, he cannot seem to get rid of her, especially when she settles herself in his apartment. Ramona proves to be irritating for the entire group, especially when she assumes that Penny likes Sheldon. She still asks Penny to be sisters with her. She makes it difficult for anyone to communicate with him and scares them away. Eventually, Sheldon solves the problem he has been working on. Enthusiastic about his solution, he asks Ramona how he can reward her for all her help. But when she asks for him to share credit with her and name the theorem "Cooper-Nowitzki", he finally kicks her out of the apartment.
Ramona reappears in the Season 10 finale. She now has her Ph.D., and has secured a post-doc position at Caltech. Amy is away at Princeton, and the gang becomes worried for Sheldon because Ramona's crush on him begins again. She brings him meals, and eventually kisses him in his office. Finally aware of her advances, Sheldon makes an excuse to leave the office, and promptly flies to New Jersey where he proposes to Amy.
In the first episode of Season 11, Amy shocks Ramona by hugging her, as it was Ramona's advances that prompted Sheldon to propose.
Ruchi (Swati Kapila); is a new co-worker of Bernadette's at Zangen who first appears in season 11. Ruchi used to live in India, and was in an arranged engagement, until she broke off the engagement and moved to America.
In her first appearance, Bernadette takes Ruchi out for drinks, though despite Howard's suggestion, refuses to set her up with Raj or Stuart. While out for drinks, Raj and Stuart show up uninvited and both take an interest in Ruchi and try to win her over. Raj invites Ruchi for drinks behind Stuart's back in a bid to get her alone. When Stuart shows up and tells Ruchi, she tells them that she is not interested in a relationship and just wants to make some friends. Raj later bumps into Ruchi at a bar while watching a cricket match with Howard and they eventually hook up. Afterwards, Ruchi wants to keep things casual with Raj, which he has difficult maintaining, especially after a date with her when she reveals that she does not believe in romantic love, and sees it more as a science. Despite this, Raj continues to see Ruchi to have sex with her. While she is on bed rest, Bernadette becomes convinced that Ruchi is trying to steal her job, which it turns out that she is, after revealing she is taking over all of Bernadette's projects. Raj tries to defend Ruchi like a boyfriend but she ends up dumping him.
Wil Wheaton plays a fictionalized version of himself. Wil played Wesley Crusher in Star Trek: The Next Generation, a character that Sheldon idolized as a child. In 1995 Wheaton made Sheldon's "mortal enemy" list after Wheaton failed to show up at a fan convention. Sheldon took a 10-hour bus ride to attend the convention and was disappointed that Wheaton was a no-show.
In season three, Sheldon enters a trading-card game tournament (Mystic Warlords of Ka'a) to confront Wheaton. When Sheldon is about to defeat his hated rival, Wheaton tells him that he missed the 1995 "Dixie Trek" convention because his grandmother had died. In sympathy, Sheldon deliberately messes up his next move to benefit Wheaton. Wheaton reveals that he lied about his grandmother and then wins the game by taking advantage of the opening Sheldon leaves him. Wheaton later plays in a bowling match against Sheldon, where he uses gamesmanship again, breaking up Leonard and Penny to win.
In season four, Wheaton appears at a special screening of Raiders of the Lost Ark and uses his celebrity status to go to the front of a long line. This antagonizes Sheldon, who derides him as the "Jar Jar Binks of the Star Trek universe". Wheaton and his entourage effectively prevented Sheldon and his friends from attending the screening when the last seat in the theater went to the person directly in front of them in line. Ultimately, Sheldon steals the reels containing the prints of the film and Wheaton leads a mob of angry audience members after Sheldon.
In season five, Wheaton invites the guys to a party at his house and appears to be genuinely sorry for his past actions, giving Sheldon a signed Wesley Crusher action figure as a way of thanking Sheldon for being a fan. His gesture is met with a renunciation of his "mortal enemy" status and a hug from Sheldon. Wheaton is invited to Howard's bachelor party, where he films the toasts and uploads them to the Internet, getting Howard and Raj in trouble with Bernadette.
In season six, Wheaton appears in a Star Trek-themed episode of Sheldon's web-series Fun with Flags and insults Amy after she criticizes his performance. After Sheldon ditches Amy to spend time with Wheaton, Amy becomes angry with Sheldon, who later unknowingly becomes intoxicated and almost starts a fist fight with Wheaton, demanding an apology.
In season seven, Amy calls Wheaton to cheer up Sheldon after Leonard disproves Sheldon's scientific discovery. Wheaton later gives Penny advice when she wrestles with whether to take a role in the sequel to a low-budget horror film in which she once starred. After taking the role, Penny finds that Wheaton is also appearing in the film. Wheaton and Penny are both fired from the film after an argument with the director.
In season eight, Wheaton invites Penny to appear on his podcast, where she talks about her acting career. Leonard and Penny get into an argument on air, much to the amusement of Wheaton.
In season nine, Wheaton introduces Adam Nimoy, son of Mr. Spock portrayer Leonard Nimoy, to Sheldon, for Sheldon to be interviewed for a documentary that Adam Nimoy is producing about Spock and his impact on popular culture. In a later episode, Wheaton fills in for Sheldon when he joins Leonard, Howard, and Raj for the midnight premiere of Star Wars: The Force Awakens. When he arrives at the theater, the audience immediately jeers him for dressing as Spock, but Wheaton explains that he did so intentionally because he believes the audience should not take whether the film succeeds or fails so seriously. He appears again to celebrate Sheldon's birthday in the show's 200th episode, "The Celebration Experimentation".
In season eleven, Sheldon wants to replace Arthur Jeffries in the role of Professor Proton, following Jeffries' death, and asks Wheaton to be his acting coach for the part. However, when Wheaton somehow gets the part instead, Sheldon is dismayed and after many unsuccessful attempts to get his friends to boycott the franchise, he demands that Wheaton turn down the role. Wheaton refuses, claiming he wants to inspire young scientists and Sheldon begins to consider him an enemy again. However, after watching the show Sheldon likes Wheaton's portrayal of Prof. Proton and gets jealous when he sees that Wheaton has brought Howard to be a guest on his show. Although Sheldon apologizes to Wheaton, mending their friendship, Wheaton refuses to bring Sheldon as a guest on his show and instead invites Amy to be a guest on the show.
Wil Wheaton, LeVar Burton, Leonard Nimoy (in voice-over only), Brent Spiner, and George Takei have all appeared on the show, making the Star Trek franchise the most represented franchise on the show in terms of guest appearances.
Mrs. Debbie Malvina Wolowitz (voiced by Carol Ann Susi) is Howard's over-protective, controlling, belittling, but loving and caring stereotypical Jewish mother. The character is not shown on-screen with the exception of one episode in season six in which her body, but not face is briefly shown. Mrs. Wolowitz's raspy voice is heard usually in scenes at her house, which she shares with Howard. She talks to Howard always by yelling at him from another room, which results in awkward conversations with Howard yelling back at her. In a flashback, when the guys hear her yelling in a masculine voice, they ask Howard if that was his father, to which Howard replies, "If she grows any more hair on her face, yes".
Mrs. Wolowitz is also described as being enormously obese and wearing a wig and painted-on eyebrows. She seems to be oblivious to Howard's work as an engineer and treats him as though he is still a child. She is a master at Wheel-of-Fortune and guessing answers at game shows. She frequently refers to Leonard, Sheldon, and Raj as Howard's "little friends" (as in, "I made some cookies and Hawaiian Punch for you and your little friends!") and often refers to Caltech as a "school". Howard often makes references to his mother as being a "crazy old lady", but deep down, he is deeply attached to his mother and quite happy to be treated like a child. She is also noted to be on hormone replacement therapy, and she wants to "play doctor" with Raj when he asks to sleep over.
When Howard reveals that he has proposed to Bernadette and she has accepted, she collapses, causing Howard to panic and dislocate his shoulder while trying to barge into the bathroom to help her. The timing leads him to believe that she may not like Bernadette (who is Catholic) and has suffered a heart attack from the shock of the news. In actuality, her fainting was due to food poisoning from the food she ate while meeting Bernadette and was completely unrelated to Howard's news. She has graciously accepted Bernadette as her daughter-in-law and is impressed by Bernadette's doctorate.
In an early episode, Howard attempts to move out after a fight with his mother, but has serious delusions that she is trying to force him to stay, when in actuality, she is urging him to move out, which leads to him staying. However, at the beginning of season six, she is angry and upset about Howard moving in with Bernadette that she cries at the thought of being lonely, prompting him to promise he'll convince Bernadette to move into their house instead. However, Bernadette remains firm but once Howard has finally moved out of his mother's house, he tells her a story about inviting his mother to be an assistant during his magic tricks as a child, helping her forget her loneliness, making Bernadette feel guilty and decide that she and Howard should sleep over at his mother's house from time to time to remind her that she is not alone. Later in the season, Howard asks Raj to look in on his mother to make sure she is okay while he and Bernadette are out on a trip. Raj enjoys dinner with her but as he is about to leave, she cries at being lonely, causing a sympathetic Raj to stay for dessert. He ends up sleeping in Howard's old bedroom, but once Howard warns him that his mother will use her loneliness and mother him in a desperate attempt to make him stay, he attempts to escape but is caught by her.
In season seven, Mrs. Wolowitz breaks her leg and keeps driving away the nurses hired to care for her. Even Penny rejects the job of caregiver after a very short time. After Stuart's comic-book store burns down, Howard invites Stuart to work for Mrs. Wolowitz; however, they develop a strange relationship, much to Howard's annoyance. Howard is also upset about the fact that now she dotes more heavily on Stuart, getting him premium cable, new clothes, money to rebuild the comic book store and even recording an answering machine message with him.
Following the death of Carol Ann Susi, producers decided that Mrs. Wolowitz would die in the season-eight episode "The Comic Book Store Regeneration". Howard receives a phone call while in the store from his aunt Gladys in Florida, whom Mrs. Wolowitz had gone to visit. She says that Mrs. Wolowitz lay down for a nap and never woke up afterwards. Howard and Bernadette then move back into Howard's old home, and Stuart decides to stay with them. He eventually moves out halfway through season nine, allowing them to redecorate the house.
Wyatt (Keith Carradine) is Penny's father. He comes to visit, and Penny feels compelled to involve Leonard in a ruse to make him think they are still together. (Wyatt reveals that he favors the successful Leonard over Penny's other "loser" boyfriends.) When the plot is revealed, he gets mad at Penny and feigns anger at Leonard, but after Penny leaves the room, he encourages Leonard to keep trying to get Penny back (because he wants his grandchildren to "grow up in a house without wheels"). Strangely, though, in the first episode where Leonard's mother visits, Penny tearfully reveals her rough childhood, saying that Wyatt wanted a boy and even called her "slugger" until she reached puberty. Wyatt appears again in season nine, when Penny finally plucks up the courage to tell her father she eloped with Leonard. Wyatt is happy, but he also confesses that he killed Penny's pet pig a year ago. In the season 10 premiere, "The Conjugal Conjecture", he comes to Pasadena with his wife and son to attend Leonard and Penny's re-wedding and during the ceremony he walks Penny down the aisle, welcomes Leonard to their family and advises him not to lend Penny's brother any money.
Zack Johnson (Brian Thomas Smith) is a dim-witted but friendly beefcake-type whom Penny dates on and off after her second break-up with Leonard. Zack works as the menu designer for restaurants that are owned by or which have hired his father's company to design their menus.
In season three, Leonard invites Penny and Zack up to the roof to watch his experiment of bouncing laser beams off the moon; Zack misunderstands what is happening, and thinks they are going to blow up the moon. Later, Penny and Zach go out to a party, but she is so put off at how stupid he is that she runs back to Leonard for sex, saying he ruined her ability to tolerate stupid men.
In season four, Zack runs into Penny, Bernadette, and Amy at a restaurant, and Amy becomes physically attracted to him. Sheldon arranges a meeting between Zack and Amy to satisfy her "urges", but his lack of intellect immediately discourages Amy. Unlike Penny's other boyfriends, Zack considers Leonard, Howard, Sheldon, and Raj (whom he calls "the science dudes") to be "cool" and makes a genuine effort to get to know them, even treating them as his friends. After accompanying them on a trip to the comic-book store (where Raj and he bond over a mutual fondness for Archie comics), he enters a New Year's Eve costume party with them, portraying Superman in their version of the Justice League. In "The Toast Derivation", Sheldon invites him over for a party, where he bonds with Kripke and Stuart, as well.
In season seven, he was unknowingly married to Penny for three years after the two were intoxicated and attended a wedding chapel in Las Vegas (they were not aware that weddings in Las Vegas are real). Penny invites him to Mrs. Wolowitz's house for Thanksgiving, where they get the marriage annulled.
In season nine, episode 22, Zack's observations indicate he is, in some respects, more observant and smarter than "the science dudes".
In season ten, episode 22, Zack and his fiancé plan a double date with Penny and Leonard but his fiancé does not turn up. Zack offers Penny a job at his menu printing company which she accepts, but when she calls Zack he tells her that his fiancé is uncomfortable with them working together.
- Dr. Alfred Hofstadter (Judd Hirsch): Leonard's father, an anthropologist who once worked with famed archaeologist Louis Leakey (Leonard's middle name is Leakey in consequence). He was miserable in his marriage to Leonard's mother Beverly, who was always annoying and was unmotherly towards their children. Alfred claimed she was cold and emasculating, and had not shown him any physical affection for years. When Leonard was six, he walked in on his parents naked, and she was swatting his rear end with Leonard's brand-new ping-pong paddle, and as a result in season three, the two were revealed to be getting divorced after he had had an affair with a waitress. In his first appearance in "The Convergence-Convergence" during his visit for Leonard and Penny's rewedding, he meets Mary Cooper and the two get along great as they have each other's nice, gentle, and caring personalities, bonding over their mutual dislike of Beverly, and their admiration of each other's beliefs. The two of them end up fighting with Beverly at the restaurant during the pre-ceremony dinner and the two leave together and after finding out they are staying at the same hotel, they decide to share a cab and it is implied that the two hook up. In "The Conjugal Conjecture", Alfred is revealed to have not slept with Mary, but they still plan on seeing each other in the future. In the ceremony, he says that though Beverly and he do not bring the best out in each other, something wonderful did come out of their relationship: Leonard. Beverly agrees. On the way to the airport, they appear to be civil for about a minute until they start bickering over Leonard's driving.
- Althea (Vernee Watson): makes an appearance in the pilot episode as an attendant at a sperm bank, and then makes three appearances as an emergency room nurse in "The Peanut Reaction" episode (season one) and "The Robotic Manipulation" episode (season four), and as a ward nurse in "The Werewolf Transformation" (season five). The name of the character is never used on the show, but is included on her name tag and in the credits. She is also the only character besides Leonard and Sheldon to be carried over from the original unaired pilot.
- Dr. Stephanie Barnett (Sara Rue): a medical doctor and a highly distinguished surgeon that Howard tries to seduce, but she falls for Leonard. She dates Leonard, and surreptitiously moves in with him. Once Sheldon and Penny point this out to him, Leonard breaks up with her.
- Captain Sweatpants (Ian Scott Rudolph) and Lonely Larry (Owen Thayer): are sometimes seen at the guys' favorite comic-book store. Captain Sweatpants is a middle-aged bald man who wears grey sweatpants and a City of Heroes T-shirt. Lonely Larry wears a brown suit and is extremely thin. They are also friends with Wil Wheaton. Both attend Howard's bachelor party.
- Cheryl (Erin Allin O'Reilly), an attendee at Penny's Halloween party in season one, episode six, "The Middle Earth Paradigm". Cheryl is the very talkative, short-haired brunette dressed as a ladybug, who shows interest in Raj. She is not identified within the episode, but her name is listed in the credits. She appears again as a Cheesecake Factory waitress in "The Pancake Batter Anomaly".
- Dale (Josh Brener), a replacement for Stuart at the comic-book store when Stuart goes on a date with Amy in "The Flaming Spittoon Acquisition". Also appears in "The Tangible Affection Proof" at Stuart's Valentine's Day party.
- Jimmy Speckerman (Lance Barber), a former high school classmate of Leonard who used to bully Leonard during their high school days on a regular basis. He comes to visit with Leonard, not realizing that his actions in high school constituted bullying.
- Dimitri Rezinov (Pasha Lychnikoff), a Russian cosmonaut who is Howard's colleague on his Soyuz mission to the International Space Station.
- Dr. David Underhill (Michael Trucco), a Macarthur Grant recipient and a successful physicist in the episode "The Bath Item Gift Hypothesis"
- Halley Wolowitz (Pamela Adlon, voice only), Howard and Bernadette's baby daughter, born in the episode "The Birthday Synchronicity". Halley's appearances are hidden by her baby pram and covered up by blankets, though her loud cries from off-screen sound a lot like Mrs. Wolowitz (a tribute to Carol Ann Susi).
- Neil Michael Wolowitz, Howard and Bernadette's baby son, born in the episode "The Neonatal Nomenclature". Hours before his birth, Howard is unhappy that Bernadette has already decided to name the baby after her father, Michael, without consulting him first. He doesn't agree with this and is even more disturbed that many of the names he has thought of come from her ex-boyfriends in college. Eventually Howard and Bernadette compromise as she gives birth to Neil, after Neil Armstrong and Neil Diamond, with Michael as the middle name. Bernadette still wants to call the baby Michael as a nickname, and Howard doesn't argue with her because of the baby's large head.
- Abby (Danica McKellar) and Martha (Jen Drohan): two ladies that Raj and Sheldon meet at a university mixer. Abby takes a liking to Raj and Martha takes a liking to Sheldon. While Raj is welcoming of Abby's moves, Sheldon is rejective of Martha's moves.
- Alice (Courtney Ford): an attractive comic book enthusiast whom Leonard meets after Priya moves back to India. Meeting Alice triggers a chain of events which ends with Leonard and Priya breaking up.
- Mandy Chao (Melissa Tang): a colleague of Leonard at the university who made out with him once when they were both drunk. The incident, which happened prior to Leonard's marriage to Penny, almost sabotages the marriages after Leonard tells Penny about it.
- Chen (James Hong): a maitre d' at a Chinese restaurant that the guys eat at.
- Professor Crawley (Lewis Black): a former entomologist at the university with whom Sheldon, Howard, and Raj confer about a cricket.
- Issabella Maria Concepcion (Maria Canals-Barrera) a Cuban-American janitor working in Raj's telescope building whom Raj wants to date.
- Kenny Fitzgerald (Michael Rapaport): an intelligent thief who sells stolen liquid helium to Leonard and Sheldon. He later strikes up a friendship with the duo and they watch Ernest Goes to Jail together.
- Mrs. Fowler (Annie O'Donnell; Kathy Bates): Amy's mother, who is played by Annie O'Donnell in "The Desperation Emanation", and by Kathy Bates in "The Bow Tie Asymmetry".
- Dr. Gallo (Jane Kaczmarek): a therapist to whom Penny attempts to sell products. She winds up conducting therapy on both Penny and Leonard.
- Mrs. Latham (Jessica Walter): a donor for the university who goes out on two dates with Leonard.
- Toby Loobenfield (DJ Qualls): a scientist at CalTech whom Sheldon hires to play his fictional drug-addicted cousin Leo.
- Dr. Oliver Lorvis (Billy Bob Thornton): a medical doctor to whom Penny sells products. He misreads her flirtations and locks the guys in his basement while he goes to woo her (as well as Amy and Bernadette).
- Meemaw/Constance Tucker (June Squibb): Sheldon's grandmother from Texas. She takes an immediate dislike to Amy upon meeting her. It appears she came to visit her grandson, but the real reason she came was to size Amy up and she dislikes Amy because of her breaking Sheldon's heart when she broke up with him. Meemaw keeps this to herself until dinner where she reveals this in front of her grandson, his girlfriend and the married Hofstadter couple. Later, Sheldon reasons with her about how she made her husband (Sheldon's maternal grandfather) a better person when he was stubborn and egotistical and Meemaw soon gives her blessings to Sheldon and Amy's relationship.
In the prequel series Young Sheldon, Meemaw is played by Annie Potts. In that series, it is revealed that she is Sheldon's maternal grandmother and has a compulsive gambling habit.
- Missy Cooper (Courtney Henggeler): Sheldon's attractive twin sister who appears in the episode "The Porkchop Indeterminacy". She is intellectually and socially quite unlike her brother. Confident, tall, and attractive, she promptly catches the attention of Leonard, Howard, and Raj. Sheldon realizes that, within Missy's eggs, lies the potential for another "superior mutation" like him. He temporarily sets himself up as a guardian to Missy in order to choose the best mate for her. Missy refers to Sheldon as "Shelly" and, despite their differences, she loves him and is proud of his accomplishments. In "The Cooper Extraction", she gives birth to a baby boy with Sheldon helping while her husband is recovering from a motorcycle accident, but does not appear on-screen. Henggeler reprises the role in the eleventh season finale "The Bow Tie Asymmetry", where Missy, pregnant with her second child, travels with her mother to Pasadena to attend Sheldon's wedding. It is mentioned that she is separated from her husband, which piques Raj's interest.
The 9-year-old version of the character is played by Raegan Revord in the prequel series Young Sheldon.
- Octavia (Octavia Spencer): a DMV clerk with whom Sheldon interacts.
- Agent Angela Page (Eliza Dushku): an FBI agent who was asked to interview Howard's acquaintances to determine his eligibility to use the Defense Department Laser Equipped Surveillance Satellite team.
- Elizabeth Plimpton (Judy Greer): an acquaintance of Sheldon's, who upon visiting, engages in flirtatiously sexual activities with Leonard, Howard, and Raj.
- Randall (Jack McBrayer): Penny's brother who first appears in the Season 10 premiere "The Conjugal Conjecture" when he comes to attend his sister's re-wedding to Leonard. He has been in and out of prison and a bit of trouble over the years.
- Sandy (Yeardley Smith) a clerk at a job bank with whom Sheldon interfaces. The purpose for Sheldon's visit is find a menial part-time job to clear his head.
- Spock (Leonard Nimoy, voice only): Appears in Sheldon's dream
- Susan (Katey Sagal): Penny's mother who first appears in the Season 10 premiere "The Conjugal Conjecture" when she comes to attend her daughter's re-wedding to Leonard. Sagal and Cuoco previously appeared in the main cast of 8 Simple Rules for Dating My Teenage Daughter. Sagal plays Cuoco's mother in that series as well.
- Trevor (Blake Anderson): an individual with whom Sheldon has conflict when he cuts in front of them at the movie theatre.
- Josh Wolowitz (Matt Bennett): Howard's half brother.
- Theodore (Christopher Lloyd): Sheldon rented out his room to Theodore in "The Property Division Collision".
- Denise (Lauren Lapkus): An employee working at Stuart's comic book store with an amazing knowledge of comic books.
- George "Georgie" Cooper Jr. (Jerry O'Connell): Sheldon and Missy's brother, who lives in Texas and runs a car tire business. Often at odds while growing up, the Cooper brothers begin to reconcile their differences when Sheldon and Leonard visit George in season 11 before Sheldon's wedding. Known as Georgie in his younger days, he appears as a main character on Young Sheldon played by Montana Jordan.
Guest stars appearing as themselves
A handful of Hollywood celebrities as well as famous scientists and engineers have made appearances on the show as themselves. In most cases the appearances are brief cameo appearances. These appearances are in contrast to Wil Wheaton who, as described above, plays a character on the show which is a fictionalized version of himself.
- Buzz Aldrin: "The Holographic Excitation"
- LeVar Burton: "The Toast Derivation", "The Habitation Configuration", "The Champagne Reflection"
- Ellen DeGeneres: "The Geology Elevation"
- Nathan Fillion: "The Comic Book Store Regeneration"
- Carrie Fisher: "The Convention Conundrum"
- Ira Flatow: "The Vengeance Formulation" (voice only), "The Discovery Dissipation", "The Retraction Reaction"
- Neil Gaiman: "The Comet Polarization"
- Summer Glau: "The Terminator Decoupling"
- Brian Greene: "The Herb Garden Germination"
- Stephen Hawking: "The Hawking Excitation", "The Extract Obliteration", "The Relationship Diremption", "The Troll Manifestation", "The Celebration Experimentation", "The Geology Elevation", "The Proposal Proposal"
- James Earl Jones: "The Convention Conundrum"
- Stan Lee: "The Excelsior Acquisition"
- Howie Mandel: "The Re-Entry Minimization"
- Mike Massimino: "The Friendship Contraction", "The Countdown Reflection", "The Decoupling Fluctuation", "The Re-Entry Minimization", "The Table Polarization", "The First Pitch Insufficiency"
- Elon Musk: "The Platonic Permutation"
- Adam Nimoy: "The Spock Resonance"
- Bill Nye: "The Proton Displacement"
- Samantha Potter: "The Panty Piñata Polarization"[b]
- Katee Sackhoff: "The Vengeance Formulation", "The Hot Troll Deviation" (both appearances are in a daydream scene)
- Charlie Sheen: "The Griffin Equivalency"
- Kevin Smith: "The Fortification Implementation"
- George Smoot: "The Terminator Decoupling"
- Brent Spiner: "The Russian Rocket Reaction"
- George Takei: "The Hot Troll Deviation" (appears in a daydream scene)
- Analeigh Tipton: "The Panty Piñata Polarization"[c]
- Neil deGrasse Tyson: "The Apology Insufficiency"
- Adam West: "The Celebration Experimentation"
- Steve Wozniak: "The Cruciferous Vegetable Amplification"
- Mark Hamill: "The Bow Tie Asymmetry"
- Although not seen or heard prior to season 4, President Siebert's name is mentioned in season 2.
- Appearance is credited, but did not have a speaking role
- Tipton makes an appearance in the Season 9 episode The Mystery Date Observation as a minor character named Vanessa Bennett who correctly solves Sheldon's challenge, but misses the deadline by 15 seconds.
- "The Middle Earth Paradigm". The Big Bang Theory. Season 1. Episode 6. October 29, 2007. 5:18 minutes in.
- "The Boyfriend Complexity". The Big Bang Theory. Season 4. Episode 9. November 18, 2010. 3:30 minutes in.
- "The Apology Insufficiency". The Big Bang Theory. Season 4. Episode 7. November 4, 2010. 12:50 minutes in.
- "The Habitation Configuration". The Big Bang Theory. Season 6. Episode 7. November 8, 2012.
- "Conlangs". Omniglot. Retrieved 11 April 2018.
- "The Countdown Reflection". The Big Bang Theory. Season 5. Episode 24. May 10, 2012.
- "The Vegas Renormalization". The Big Bang Theory. Season 2. Episode 21. April 27, 2009. 9:42 minutes in.
- "The Psychic Vortex". The Big Bang Theory. Season 3. Episode 12. January 11, 2010. 8:25 minutes in.
- "The Grasshopper Experiment". The Big Bang Theory. Season 1. Episode 8. November 12, 2007.
- "The Middle Earth Paradigm". The Big Bang Theory. Season 1. Episode 6. October 29, 2007.
- Ausiello, Michael (January 23, 2009). "Big Bang Theory scoop: Sara Gilbert taken off contract". Entertainment Weekly. Retrieved November 5, 2016.
- "The Herb Garden Germination". The Big Bang Theory. Season 4. Episode 20. April 7, 2011. 19:08 minutes in.
- "The Stag Convergence". The Big Bang Theory. Season 5. Episode 22. April 26, 2012. 07:41 minutes in. CBS.
- "The Commitment Determination". The Big Bang Theory. Season 8. Episode 24. May 4, 2015. Event occurs at 13:35. CBS. Howard tells Bernadette his middle name, because he wants her to make him move out of his mother's house.
- "The Flaming Spittoon Acquisition". The Big Bang Theory. Season 5. Episode 10. November 17, 2011. Event occurs at 11:44. CBS. Sheldon is viewing Stuart's Facebook page, where his name is shown as "Stuart Bloom".
- "The Comet Polarization". The Big Bang Theory. Season 11. Episode 21. April 19, 2018. CBS.
- "The Hofstadter Isotope". The Big Bang Theory. Season 2. Episode 20. April 13, 2009. CBS.
- Andreeva, Nellie (August 22, 2012). "Big Bang Theory 's Kevin Sussman Upped To Regular". Deadline Hollywood. Retrieved November 5, 2016.
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- Sleasman, MaryAnn (January 11, 2013). "The Big Bang Theory "The Bakersfield Expedition" Review: Comic Book Women". TV.com].
- "The Killer Robot Instability". The Big Bang Theory. Season 2. Episode 12. January 12, 2009.
- "The Friendship Algorithm". The Big Bang Theory. Season 2. Episode 13. January 19, 2009.
- "The Vengeance Formulation". The Big Bang Theory. Season 3. Episode 9. November 23, 2009.
- "The Toast Derivation". The Big Bang Theory. Season 4. Episode 17. February 24, 2011.
- "The Bat Jar Conjecture". The Big Bang Theory. Season 1. Episode 13. April 21, 2008.
- "The Griffin Equivalency". The Big Bang Theory. Season 2. Episode 4. October 13, 2008.
- "The Cornhusker Vortex". The Big Bang Theory. Season 3. Episode 6. November 2, 2009.
- "Pilot". The Big Bang Theory. Season 1. Episode 1. September 24, 2007.
- "The Middle-Earth Paradigm". The Big Bang Theory. Season 1. Episode 6. October 29, 2007.
- "The Financial Permeability". The Big Bang Theory. Season 2. Episode 14. February 2, 2009.
- Young Sheldon episode "Jiu-Jitsu, Bubble Wrap, and Yoo-Hoo"
- "The Panty Piñata Polarization". The Big Bang Theory. Season 2. Episode 7. November 10, 2008.
- "The Irish Pub Formulation". The Big Bang Theory. Season 4. Episode 6. October 28, 2010. CBS.
- "The Infestation Hypothesis". The Big Bang Theory. Season 5. Episode 2. September 22, 2011. CBS.
- "The Good Guy Fluctuation". The Big Bang Theory. Season 5. Episode 7. October 27, 2011. CBS.
- "The Status Quo Combustion". The Big Bang Theory. Season 7. Episode 24. May 15, 2014. 20:01 minutes in. CBS.
- "The Birthday Synchronicity". The Big Bang Theory. Season 10. Episode 11. December 15, 2016. CBS.
- "The Lunar Excitation". The Big Bang Theory. Season 3. Episode 23. May 24, 2010.
- Prudom, Laura (February 3, 2016). "'Big Bang Theory' 200th Episode: Wil Wheaton, Adam West Among Guest Stars". Variety. Retrieved February 3, 2016.