List of The Big Bang Theory characters
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The American sitcom The Big Bang Theory, created and executive produced by Chuck Lorre and Bill Prady, premiered on CBS on September 24, 2007. The show centers on five characters: roommates Sheldon Cooper and Leonard Hofstadter, 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 Rajesh Koothrappali.
Over time, several supporting characters have been promoted to starring roles: Leslie Winkle (season 1 recurring; season 2 regular; season 3 guest), a physicist colleague, and, at different times, a lover of both Leonard and Howard; Bernadette Rostenkowski-Wolowitz (season 3 recurring; season 4-present regular), a microbiologist, Howard's fiancée and subsequent wife, and former part-time waitress alongside Penny; neuroscientist Amy Farrah Fowler (season 3 guest; season 4 recurring; season 4-present regular), who joins the group after being matched to Sheldon on a dating website; and comic book store-owner, Stuart Bloom (seasons 2–5 recurring; season 6-present regular), who replaces Howard as Raj's best friend when Howard is in space. The show also features numerous supporting characters, each of whom plays a prominent role in a small group of episodes.
Main characters 
Main characters are divided into two groups, original main characters and those who have been promoted to main character status since season two.
Original main characters 
With the exception of injury to the actors, these characters have appeared in all episodes.
Leonard Hofstadter 
Dr. Leonard Leakey Hofstadter (Johnny Galecki) is an experimental physicist with an IQ of 173 originally from New Jersey who received his PhD when he was 24-years old. He shares an apartment in Pasadena with colleague and friend Dr. Sheldon Cooper. Unlike his equally geeky friends, Leonard is interested and quite adept in building interpersonal and social relationships with others, and is quite successful with women. The writers have toyed with a romance between him and next-door neighbor Penny since the pilot, with their unresolved sexual tension being a major force for drama. Leonard dated Penny for most of Season 3, although they had dated briefly at the end of Season 1. Aside from Penny, Leonard has had romantic relationships with co-worker Dr. Leslie Winkle, physician Dr. Stephanie Barnett, an unnamed French literature PhD, and North Korean spy Joyce Kim. Following his breakup with Penny, Leonard began a relationship with Raj's sister Priya in Season 4, and it is implied they had a brief sexual encounter before the start of the series on an occasion when Priya visited Raj. The two attempted a long-distance relationship after Priya moved back to India, but Leonard eventually broke up with her in Season 5 after she admitted to sleeping with her old boyfriend. In Season 5's "The Beta Test Initiation", Leonard and Penny start dating anew though their relationship is rocky. Leonard's family includes other accomplished scientists: his mother, Dr. Beverly Hofstadter, is a world-renowned psychiatrist, his sister is a biologist and his father an anthropologist. Also, his brother Michael is a Harvard law professor. It is mentioned in the season 2 episode "The Maternal Capacitance" that Leonard is the least successful member of his family. He is also known to be lactose intolerant and cannot process corn, as Sheldon states in "Pilot". Leonard also plays the cello.
Sheldon Cooper 
Dr. Sheldon Lee Cooper (Jim Parsons) is a theoretical physicist, possessing a Bachelor of Science, Master of Science, Master of Arts, a PhD, an ScD, and an IQ of 187. Originally from East Texas, he was a child prodigy, starting college at the age of 11, and receiving his PhD at age 16. He is cold and calculating, and refrains from any form of physical contact if possible. Sheldon exhibits a strict adherence to routine to the point of exhibiting obsessive–compulsive disorder, performing specific activities on specific days and specifics times, such as doing the laundry on Saturday night at 8:15. Any disruption to this routine distresses him enormously. A running gag has him always knocking on doors or other surfaces in a pattern of three knocks and one call repeated three times, which he has to complete no matter what. He has forced Leonard into signing an absurdly exhaustive "Roommate Agreement", and later enters a similar "Relationship Agreement" with Amy Farrah Fowler. Sheldon has great difficulty identifying sarcasm and irony and responding in kind, and a recurring topic is his efforts to acquire these skills. He has also enormous difficulties lying or even keeping a secret, giving himself away with wild facial tics and implausible statements. He is however fond of occasionally playing a prank, which he typically punctuates it with his trademark exclamation "Bazinga!". Sheldon has a superiority complex and makes no effort to hide his contempt for other people's intellects, often making highly inappropriate comments. A common joke is his inability to drive, although he did receive a learner's permit. Sheldon loves comic books and science fiction like his friends, but to the point of being obsessed about them, and is a fan of Spock, a fictional character of the Star Trek franchise. Unlike Leonard and Raj who come from intellectual families, Sheldon stands out from his own relatives. His father, George, whose death pre-dates the series, was described as a stereotypical Texan who liked football, skeet shooting, and heavy drinking, his mother, Mary, is a devout Evangelical Christian, who does not possess anything near Sheldon's intellect (though she displays a great deal of common sense); his twin sister, Missy, and as-yet unseen brother, George Jr., were both described by Mary as being "dumb as soup". He meets Amy Farrah Fowler in the Season 3 finale and starts a highly unusual and very slowly evolving relationship with her. Previous to this, he has also exhibited some affinities with Leonard's equally unemotional mother.
Penny (family name currently unrevealed) (Kaley Cuoco), is Leonard and Sheldon's neighbor across the hallway. Originally from a small town outside of Omaha, Nebraska, she is a waitress and occasional bartender at the local Cheesecake Factory and also an aspiring actress. Her profession and educational qualifications (she had 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 love interest is Leonard. Leonard pursued Penny during season 1; they dated briefly at the start of season 2 which led to some awkwardness; they began dating again and were a couple for most of season 3 but she broke up with him after he told Penny he loved her and she was not ready to reciprocate. Penny and Leonard begin to date again "slowly" during the fifth season, though he does upset her by impulsively asking her to marry him during sex. She has a tense relationship with Sheldon, whose nerdy and obstinate personality often exasperate her, but they are ultimately good friends, helping each other out in various situations. By season four, Penny also begins to socialize more with Bernadette and Amy, who frequently hang out in Penny's apartment, go out together and comfort each other. Bernadette and Amy are both shown to be simultaneously envious of and threatened by Penny's more outgoing personality. Not much is known about Penny's family, but it is mentioned in the series that her father, Wyatt, raised her like a boy, her mother smoked pot while she was pregnant with her, her sister shot her husband while they were drunk and her brother is a meth addict.
Howard Wolowitz 
Howard Joel Wolowitz (Simon Helberg) is an aerospace engineer at Caltech's Department of Applied Physics with a Masters in Engineering 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 season 5 finale, Howard goes to the International Space Station on Expedition 31. Howard lives in Altadena with his overbearing 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.[episode needed] 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 female is present, which often comes off as inappropriate and disgusting, although he has dropped this habit since he started going out with Bernadette.[episode needed] He is Jewish, but he is not very serious about his faith, and does not keep kosher. For example, when the group's favorite Chinese restaurant raised the price of pork, Howard remarked, "It's getting tougher and tougher to be a bad Jew." However, he does observe Sabbath and the Jewish holidays. In the pilot episode, it's shown that he is a polyglot and can speak English, French, Mandarin, Russian, Arabic, and Farsi, along with fictional languages like Klingon and Sindarin. He is asthmatic, allergic to peanuts, almonds and walnuts, prone to canker sores and pink eye, succumbs to seasickness, has an incredibly high genetic risk of heart illness, and has transient idiopathic arrhythmia. In Season 5 episode 4, it's shown that he knows American Sign Language. Howard dated Bernadette Rostenkowski briefly in season 3. After some difficulties, they resumed their relationship in season 4, which culminated in their engagement. In season 5, Howard and Bernadette are married on the roof of the apartment building the day before he has to go into space.
Rajesh Koothrappali 
Dr. Rajesh Ramayan "Raj" Koothrappali (Kunal Nayyar) is Howard Wolowitz's best friend, and yet another genius of the group; his name is usually shortened to "Raj". He is originally from New Delhi, India, and he works in the Physics department at Caltech, where his area of expertise is particle astrophysics. 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, and is usually the one to propose various real-life questions derived from such works, such as the functions of the tails of the Na'vi in the film Avatar. He is also a fan of Harry Potter. Raj hates India, Indian food and Indian music, however, he does appreciate the Indian lullabies his mother sang to him and the catchiness of Hindi phrases. Raj is a Hindu and believes in karma (reincarnation), but eats beef. Raj is extremely shy around women, and finds himself unable to speak to or while in the presence of them unless he drinks alcohol, or he believes he has drunk alcohol. When Penny or Bernadette is around, Raj usually whispers what he wants to say to Howard or Leonard, who then repeats or responds to what Raj says out loud, although at times Howard will embarrass him by saying something completely different and inappropriate.[episode needed] Despite this, Raj has sometimes ended up in bed with women, leaving the other guys perplexed.[episode needed] Unlike his friends however, Raj has many quite 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 a crush on 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, who dates Leonard in Season 4, breaking up in Season 5. After making a brave speech about not being embarrassed to be single, Raj started seeing a very shy and sweet girl named Lucy. Lucy has many things in common with Raj including her fear of being in large crowds of people.
Additional main characters 
The following characters have been promoted to main character status since season 2. Leslie Winkle and Stuart Bloom are only credited in episodes in which they appear. Bernadette and Amy Farrah Fowler were promoted to main character status in season 4, and their images were included in the main title sequence from the first episode of season 6.
|Sara Gilbert||Leslie Winkle||3||4||1|
|Melissa Rauch||Bernadette Rostenkowski||5||15||22||23|
|Mayim Bialik||Amy Farrah Fowler||1||17||21||22|
|Kevin Sussman||Stuart Bloom||2||4||3||7||8|
Leslie Winkle 
Dr. Leslie Winkle (Sara Gilbert) (appeared in eight episodes total, four credited as main cast) 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 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. Leslie has had casual sex relationships with Leonard and later Howard. Leslie's final appearance was in the season 3 finale, when Leonard, distraught after Penny, who had broken up with him, had gotten drunk and had sex with him, asked for casual sex with Leslie. Leslie responded, "let me think about it", before closing the door on him.
Dr. (after "The Roommate Transmogrification") Bernadette Maryann Rostenkowski-Wolowitz (after marrying Howard, nee Rostenkowski) (Melissa Rauch): (appeared in 64 episodes total, 59 credited as main cast) is originally a waitress and co-worker of Penny's at the Cheesecake Factory, which pays the way for her graduate studies in microbiology per "The Creepy Candy Coating Corollary". A recurring joke in the series is in reference to Bernadette handling infectious biological specimens before preparing food or making physical contact with people, which eventually results in her and her entire research team spending New Year's Eve 2010 in quarantine. She is of Polish origin and Catholic upbringing and admits that she cannot lie very well because, according to her, the teachers "kind of whip that out of you" – and is frequently seen wearing a cross necklace. Bernadette is introduced to Howard by Penny after he invoked the "Girlfriend Pact". At first, she and Howard do not get along, as they appear to have nothing in common. When they find out that they both have overbearing mothers, they feel a connection. Subsequently, Howard realizes Bernadette presents a real opportunity to develop a lasting relationship and, in an impulsive manner, he proposes to her. Although Bernadette rejects his offer, they remain a couple for a time after Howard sings a heartfelt song for her at the Cheesecake Factory, which Penny finds quite embarrassing, but Bernadette finds romantic. Eventually they break up when she discovers Howard using his avatar to have sex with another online character in World of Warcraft. Bernadette and Howard reconcile and resume their relationship, ultimately culminating in their engagement in the season 4 episode, "The Herb Garden Germination". In "The Roommate Transmogrification", she finally obtains her Doctorate and is subsequently hired by a pharmaceutical company that promises her "a butt-load of money". This makes Howard jealous, especially after the rest of his friends taunt him about his lack of a Doctorate. This eventually leads to them having an argument after she purchases a Rolex watch for Howard, which he interprets as her "rubbing in" the fact that she now earns more than him. Howard later learns that Raj fantasizes about Bernadette and writes love poems about her. Bernadette almost breaks up with Howard after hearing about his previously-unknown former sexual escapades that came out during his bachelor party from the episode "The Stag Convergence". Howard and Bernadette are married (by the entire main cast) on the roof of the apartment that day before he has to go into space. During season 6 the newlyweds do have some bumps, including Howard readjusting to being back on earth, moving out of his mother's house. and again when he spends too much on his collectibles hobby.
Amy Farrah Fowler 
Dr. Amy Farrah Fowler ( Dr. Mayim Bialik) (appeared in 61 episodes total, 55 credited as main cast) is a neuroscientist (which corresponds to Bialik's real-life Ph.D.), and Sheldon's love interest in the series. She has a Ph.D. in neurobiology, with a research focus on addiction in primates and invertebrates, occasionally mentioning such experiments as getting a capuchin monkey addicted to cigarettes or starfish hooked on cocaine. Raj and Howard met Amy through an online dating site after secretly setting up 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". They briefly ended their friendship when they got into an argument over whose field of study was superior. The two initially communicate online and via webcam, but at the encouragement of the group, she starts hanging out with the gang in person. In Season Five, after Amy went out on a date with comic book store owner Stuart, Sheldon decided to solidify their relationship as boyfriend/girlfriend with a 31-page "Relationship Agreement". Amy began a campaign in "The Launch Acceleration" to increase Sheldon's feelings for her by becoming more involved in his interests like video games and Star Trek, and treating him as his mother had. During Howard and Bernadette's wedding ceremony, Amy is disappointed when Sheldon says that he hopes that the two of them are as happy together as he is alone. Later as Howard is launched into space, Amy is surprised when Sheldon spontaneously grabs her hand. Despite her best efforts, however, he repeatedly resists all of her attempts at persuading him to engage in physical intimacy though they are regularly holding hands at Amy's insistence at the beginning of season 6. By mid-season when Sheldon was caring for her when she had the flu, Sheldon has some intimate views of Amy pushing him further toward a normal relationship with her. Amy tried to move into his apartment when Leonard temporarily vacated it. After being prodded by Barry Kripke about his girlfriend, he lied that they were having coitus, though he admitted to Penny that it might really happen between him and Amy and later admited it to Amy. At the same time, while early on she 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. More recently, she has shed her strident, aloof personality for a more feminine and social one. She is also noted for her close friendship with Penny, at one point referring to her as "bestie," and, on another occasion, admitting her entire social life revolves around her. Penny herself is fond of Amy, on one occasion making an effort to salvage their friendship after she accidentally hurt her feelings. To her dismay, however, Amy often fails to grasp the concept of "girl talk," by talking about feminine hygiene and anatomy when in her presence. Also, Amy often demonstrates an infatuation towards her, and occasionally Bernadette though she still makes condescending remarks toward Bernadette. These including making inappropriate comments about Penny's body. In episode 8 of the fifth season, she shows a disturbing enthusiasm on the whim that Bernadette's bridal shower should consist of naked bridesmaids washing each other, and she proposes that the bachelorette party consist of a Native American sweat lodge and women dancing around painting fertility symbols on the bride's naked body. Her more arrogant side is shown in "The Pulled Groin Extrapolation", where she's convinced that Leonard has fallen for her after he accompanies her to a friend's wedding and gives her a chaste kiss on the cheek, even though he has no such feelings. In "The Benefactor Factor", it is revealed that she is technically engaged to Saudi Arabian Prince Faisal who resides in Riyadh, who is the source of much of her lab's funding. Amy can also play the harp. The actress portraying Amy Farrah Fowler, Mayim Bialik, was mentioned by Raj ("the girl who played TV's Blossom") in "The Bat Jar Conjecture", as a possible substitute for Sheldon on their Physics Bowl team.
Stuart Bloom 
Stuart Bloom (Kevin Sussman) (appeared in 23 episodes total, seven credited as main cast) Stuart runs the comic book store that the guys patronize. He is a talented portraiture artist who attended the Rhode Island School of Design. During Stuart's first appearance, the guys brought Penny to the store and he managed to ask her on a date. The date went fine until Stuart was dragged into discussing comic books by Sheldon, leaving Penny bored. On a second date with Penny, Stuart asked for Leonard's advice, but feeling threatened, Leonard ultimately gave him bad advice. The date went fine, but when Penny was getting romantic with Stuart she mistakenly called him "Leonard", leaving him devastated. Stuart was the partner of Wil Wheaton in a trading card game tournament in which they defeated the team of Raj and Sheldon. As Stuart runs a comic book store, he has an almost encyclopedic knowledge of comic books and superheroes. In the Season 4 episode "The Toast Derivation", he mentions that he is in financial trouble and that the comic book store is now also his home. In "The Justice League Recombination", he states that he works 70 hours and clears an average of $1.65 per hour after expenses, so his income per week would be $115.50. At Howard's bachelor party during the "The Stag Convergence" episode, Stuart uses his toasting turn to tell Howard how lucky he is by contrast to Stuart's own existence. In the Season 6 episode "The Bakersfield Expedition," Penny asks Stuart for advice about comics, and he sullenly answers, "Don't own a store that sells them." During "The Decoupling Fluctuation", Stuart becomes close friends with Raj and starts hanging out with the gang as a replacement for Howard while Howard is in space. Before accepting this, Sheldon quizzed Stuart as to where he went to school (Stuart has an art degree). Sheldon concedes to allow Stuart to be in the group, calling him "fake-Wolowitz" and equating his past career choice in art to Howard's as an aerospace engineer as "equally ridiculous". When Howard returns from space 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. Sheldon relents after Stuart offers to go as high as 30%. Initially, Sheldon had stipulated that Stuart must act like Howard while in the gang's presence.
Recurring characters 
These characters appear in several episodes. The list is sorted by chronological order of appearance, considering their first appearance in the show.
Dr. Eric Gablehauser 
Dr. Eric Gablehauser (Mark Harelik): The head of the Physics Department, Dr. Gablehauser is the group's boss at the university. In his first appearance, he fired Sheldon after Sheldon insulted his intelligence. Eventually, Sheldon was rehired because Gablehauser became romantically interested in Mary Cooper, Sheldon's mother. Gablehauser refers to Sheldon, Leonard and Rajesh as doctors, but to Howard as "mister" because he lacks a PhD (but carries a Masters Degree). Dr. Gablehauser was also responsible for introducing child genius Dennis Kim to the university and for hosting the Physics Bowl.
Mary Cooper 
Mary Cooper (Laurie Metcalf): A devout Fundamentalist Christian from Texas, Mary is Sheldon's mother. She has two other children besides Sheldon; Sheldon's twin sister, Missy and his brother George, who is shown via a family tree in "The Cruciferous Vegetable Amplification" to be three years older than Sheldon and Missy. She appeared in the fourth episode of the first season, the season three premiere, the third episode of season four, and the sixth episode of season five. Mary herself is not intellectual but is quite wise, though she is prone to making politically incorrect comments such as referring to "kung fu letters" when Leonard takes her to a sushi restaurant, or saying that "I thought it was our Indians that had the occasional drinking problem" when she sees Raj drunk. She appears to be an extremely good mother and is the only one who has ever been able to control Sheldon. Leonard has described Mary as Sheldon's "Kryptonite". 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 was as loving as Sheldon's mother, although Sheldon himself appears at best ambivalent about her parenting. It is once mentioned by Sheldon 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!"
Mrs. Wolowitz 
Mrs. Wolowitz (voiced by Carol Ann Susi): Howard's overbearing Jewish mother, the character is not shown on-screen until the season 6 episode "The Spoiler Alert Segmentation", but her deep voice is heard when he is at their house or when he talks to her on the phone. She talks to Howard always by yelling at him from another room, which results in awkward conversations with Howard yelling back at her. In an early episode, when the guys heard her yelling in a masculine voice, they asked Howard if that was his father; to which he replied "If she grows any more hair on her face, yes"; hinting that she suffers from facial hair growth. She is also described as being enormously obese (especially in the arms), and wearing a wig and painted-on eyebrows. Mrs. Wolowitz seems to be oblivious about Howard's work as an engineer, and treats him like he is still a child. 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. However, in one episode, Howard attempted to move out after a fight with his mother, but had serious delusions that she was trying to force him to stay, when in actuality, she was urging him to move out, so he stayed. In the penultimate episode of season 4, she wants to "play doctor" with Raj when he asks to sleep over. When Howard revealed that he proposed to Bernadette and she accepted, she collapsed, causing Howard to panic (and dislocate his shoulder trying to barge into the bathroom to help her), and the timing leads him to believe that she may not have liked Bernadette and had suffered a heart attack from the shock of the news. In actuality, her fainting was, coincidentally, due to food poisoning from the food she ate while meeting Bernadette, and was completely unrelated to Howard's news. She has accepted Bernadette as her future daughter-in-law and loves that she is a doctor. She attended Howard and Bernadette's wedding on Leonard/Sheldon/Penny's apartment building roof and can be seen in the satellite picture of the event. The episode "The Spoiler Alert Segmentation" was a major episode in Mrs. Wolowitz's character as she, for the first time, appeared on screen; the first two times her figure was seen walking across the kitchen through the kitchen doors, and then in the end of the episode, her hands were seen pulling Raj back in. Her face was not seen at all, but her appearances in the kitchen scene make her out to be an extremely fat woman.
Dr. and Mrs. Koothrappali 
Dr. V. M. Koothrappali (Brian George) and Mrs. Koothrappali (Alice Amter): Raj's parents in India, they communicate with their son via video chat and constantly try to arrange dates for him. They want their son to marry a woman of Indian descent, and give them grandchildren. Mrs. Koothrappali is especially worried that, despite Raj being old enough to marry, the closest they have to a daughter-in-law is the "Jewish boy", Howard. 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 gynaecologist, drives a Bentley and employs servants, for which Raj replies that the Bentley is leased and that some of the servants are children. The Koothrappalis enjoy Doogie Howser reruns, which are apparently new to India. Other than Raj, they also maintain contact with Priya Koothrappali via video chat whenever she is out of India. They also had some anger towards Leonard when it became apparent that he was dating Priya without their knowledge.
Dr. Stephanie Barnett 
Dr. Stephanie Barnett (Sara Rue): A doctor and highly distinguished surgical resident at Fremont Memorial, Stephanie was picked up in a bar by Howard but when his plan to let her drive the Mars rover failed, she left with Leonard and the two immediately became attracted to each other. Sheldon began meddling in Stephanie and Leonard's new relationship in an effort to consolidate it, since in his eyes, Stephanie was the only "tolerable" mate Leonard had had. After several weeks of dating, Stephanie officially began living with Leonard in his and Sheldon's apartment. However, after much hesitance, Leonard told her to move out because he was uncomfortable with the pace of the relationship. They were not seen breaking up on camera. Series co-creator Bill Prady confirmed this in an interview, stating: "Stephanie was a chance for Leonard to learn that just because someone loves you, doesn't mean you'll love them back". Stephanie did her medical internship at Lawrence Memorial in Galveston, Texas, which is where Sheldon was born.
Dr. Beverly Hofstadter 
Dr. Beverly Hofstadter (Christine Baranski): Leonard's overly analytical mother, Beverly is a neuroscientist as well as a psychiatrist. With her neurotically strict speech patterns, lack of social conventions, and attention to detail, she is Sheldon's female equivalent. The two of them share an odd non-romantic attraction to each other, which culminates in her kissing him after becoming drunk. Though still married to Leonard's father, she revealed in an appearance that the two "are getting a divorce". She had frequently implied that the two have not had "coitus" since Leonard was conceived. She diagnosed Raj with selective mutism and considered Raj and Howard's relationship to be an "ersatz homosexual marriage". Beverly mentioned that Leonard's brother and sister are more successful in their respective fields than he is. Ironically, as displayed in "The Maternal Congruence" episode, Sheldon is closer to Beverly than her own son is. In both 2009 and 2010, Christine Baranski was nominated for an Emmy Award for Outstanding Guest Actress in a Comedy Series for this role. She returns via Skype for the season 5 premiere.
Barry Kripke 
Barry Kripke, Ph.D. (John Ross Bowie): An unlikable co-worker of Leonard and Sheldon's, who works in plasma physics, Kripke has a case of rhotacism where he pronounces the letters "R" and "L" as "W" in much the same way as Elmer Fudd from Looney Tunes. In his first appearance, he pitted his robot, the Kripke Krippler, against the men's robot, M.O.N.T.E., in an unofficial robot fight. On another episode, Sheldon attempted to befriend Kripke in order to gain access to an open science grid computer to carry out research, but it turned out to be futile, as Kripke had no control of the computer as Sheldon had thought. Kripke continued his antagonism towards Sheldon, when he pulled a prank on Sheldon when the latter was a guest on NPR's Science Friday. Sheldon tried to befriend Kripke another time when he was feeling left out from his friends, but Barry connected more with the others in Sheldon's new proposed group. Kripke purchases an iPhone with voice recognition technology, but due to his rhotacism, the device is incapable of understanding his verbal "wequests". In Season 5's "The Rothman Disintegration", Kripke and Sheldon battle over a retiring professor's office. At Howard's bachelor party during the "The Stag Convergence" episode, Kripke uses his toasting turn to complain that there were no strippers at the party. In the Season 6 episode "The Cooper/Kripke Inversion", Kripke and Sheldon are forced to work together on a proposal involving fusion reactors, which involves the 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, thus his work quality has declined as a result, and Sheldon doesn't deny it. Kripke likely holds a doctorate, as he is seen applying for tenure at the university in "The Tenure Turbulence."
Wil Wheaton 
Wil Wheaton (portraying a fictionalized self-serving version of himself): In the season three episode "The Creepy Candy Coating Corollary", Sheldon enters a trading card game tournament (Mystic Warlords of Ka'ah) to confront Wil Wheaton over an incident that occurred in 1995 when Sheldon was devastated because Wheaton (Wesley Crusher in Star Trek: The Next Generation) failed to show up at a fan convention that Sheldon took a 10-hour bus ride to attend (in which he had to "twice violate his personal rule of no relief on a moving vehicle"). In the final match, Sheldon is about to defeat his hated rival, but Wheaton tells Sheldon that the reason he missed the convention was because his grandmother had died. After Sheldon wastes his last opportunity to win under sympathy, Wheaton reveals that he lied about his grandmother, allowing him to win the game. Wil Wheaton makes a second appearance in the episode "The Wheaton Recurrence", where he uses gamesmanship again as seen when he breaks up Leonard and Penny's relationship in order to win a bowling competition against the main characters. In "The 21-Second Excitation", Wheaton appears at a screening of Raiders of the Lost Ark, in which he 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 prevent Sheldon and the guys from attending the screening when the last seat in the theater goes to the person directly in front of them in line. Ultimately, Sheldon steals the prints of the film and Wheaton leads a mob of angry Raiders fans after Sheldon, Howard, Raj and Leonard. In the season five episode "The Russian Rocket Reaction", Wheaton appears to be genuinely sorry for his past actions, and gives 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. Along with LeVar Burton, Leonard Nimoy, Brent Spiner and George Takei, Wheaton makes Star Trek the most represented franchise on the show in terms of guest appearances. At Howard's bachelor party during the "The Stag Convergence" episode, Wil video-records the toasts and uploads them to the Internet, getting Howard and Raj in trouble with Bernadette and showing that he hasn't lost all of his "evil" quirks. In the season six episode "The Habitation Configuration" Wheaton appears in a Star Trek-themed episode of Sheldon's webseries Fun with Flags, and insults Sheldon's girlfriend (and the webisode's director) Amy. After ditching Amy to hang out with Wheaton (described by Sheldon as "Once you explain who he is, many people recognize him."), Amy gets mad and Sheldon later becomes drunk and tries to start a fist-fight with Wheaton.
Zack Johnson (Brian Thomas Smith): A dim-witted beefcake-type whom Penny dates after her second break-up with Leonard. Leonard invites Penny and him up on the roof to watch his experiment of bouncing laser beams off the moon; Zack has no clue what is happening. Later, he and Penny go out on a date, but she is so put off at how stupid he is that she runs back to Leonard to have sex, saying that Leonard has ruined her ability to date stupid men. In a later episode, he runs into Penny, Bernadette, and Amy at a restaurant, and Amy becomes physically attracted to him. Unlike Penny's other boyfriends, he considers Leonard, Howard, Sheldon, and Raj (whom he calls "the science guys") to be "cool" and makes a genuine effort to get to know them, considering them his friends. After accompanying them on a trip to the comic book store (where he and Raj 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. He works as the menu designer for restaurants either owned or who employ his father's company to design their menus. In "The Toast Derivation", Sheldon invites him over for a party, where he bonds with Barry Kripke and Stuart as well.
Priya Koothrappali 
Priya Koothrappali (Aarti Mann): Raj's younger sister, introduced in the fourth season. Having graduated at the top of her class at Cambridge, she is one of the lead attorneys in India's biggest car company. It is disclosed upon her first appearance in "The Irish Pub Formulation" episode that she and Leonard had a relationship five years earlier (without her parents' or Raj's approval) when she was in town, and that Leonard had been much more invested in the relationship than she. Leonard even offered to relocate to Delhi to be with her. He tries to keep their relationship a secret from the guys, but Sheldon discovers Priya in their apartment. Leonard eventually reveals to the rest of the guys that he had 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 she and Leonard decide to try and 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, while Leonard is unable to do so. Priya contests Sheldon's roommate agreement with Leonard, employing her legal background to deny Sheldon power over Leonard, but Sheldon drafts a new agreement and blackmails Leonard into signing it. Sheldon then threatens to tell Priya's parents she is dating Leonard, which she cannot bear because they want her to either date an Indian or, possibly, a "nice Jewish boy like Howard." In the fourth season finale, it is revealed she is moving back to India and has not told Leonard, which Leonard interprets as the end of their relationship and causes him to expose their secret relationship to her parents. During Season 5, although they are then separated with Priya moving back a month earlier than expected, they nonetheless remain in-touch via webcam, having a cyber-dinner date/breakfast date. At Howard's suggestion, Leonard and Priya also attempted to simulate sex via webcam, but when it came time for her to remove her clothing, Leonard's screen froze up because of exceeding the bandwidth. Leonard later managed to get cue cards to help himself with a seduction speech to Priya, but while delivering it to her, he inadvertently revealed to her parents that they were cyber-dating (they were in the same room as her when he called), with them bitterly asking for him to continue with the speech. After Leonard tells Priya about his attraction to a comic book enthusiast named Alice, Priya admits to recently sleeping with her ex-boyfriend, which visibly upsets Leonard. She claims that its not a competition, to which he replies that it is and that she won. Then he switches off his laptop. In the next episode, Leonard refers to himself as "single", indicating he and Priya have broken up as a result of their last conversation.
Alex Jensen 
Alex Jensen (Margo Harshman) : An attractive Caltech graduate student hired by Sheldon to review his kindergarten and elementary school notebooks for any possible Nobel Prize winning research. Amy is at first jealous until she sees Alex instead flirting with an unresponsive Leonard. Penny is given something to think about and is mistaken for Leonard's research assistant. Later, when Leonard admits his jealousy regarding a writing partner Penny is working with in a community college class, Alex attempts to flirt with him during the conversation, then again later in a text message. Leonard appears completely oblivious to her interest, however. In a later episode that season, Leonard finally realizes that Alex is making advances toward him. While proclaiming his undying loyalty to Penny, Leonard is flattered by Alex's interest to the point of giddiness. This type of attention though, has occasionally gotten the guys into big trouble. For example in the episode titled "The Egg Salad Equivalency" after Sheldon discovered that Alex made a hit on Leonard, he told Penny, Amy, and Bernadette about the situation going on between Alex and Leonard which angered Penny because Leonard loved Alex hitting on him. Sheldon subsequently lectured Alex, which included relating his late father's somewhat offensive joke comparing women to an egg salad sandwich. This only resulted in him getting called into Human Resources after Alex felt offended during the talk and made a complaint. Sheldon then manages to offend Mrs. Davis, the African-American human resources director, in both gender and racial manners. He furthermore mistakenly told about other instances involving Leonard, Howard, and Raj. That resulted in the other three guys being called down as well and after his first meeting, Sheldon came back down and filed a complaint against Ms. Davis for making him feel offended. By coming back down to Human Resources, he scored a second meeting with Mrs. Davis that ended with Sheldon having to apologize to Alex and take a university-mandated online Sexual Harassment course. Sheldon then forced Alex to take the online course for him, due to him feeling his own time was too valuable to waste on it. Alex scores some points with Sheldon after he sends her out to buy Amy a Valentine's Day gift, and Sheldon likes it so much that he keeps it for himself.
Minor characters 
These characters appear in fewer episodes, most in just one. The list is sorted by chronological order of appearance, considering their first appearance in the show.
Althea (Vernee Watson) A character that made an appearance in the pilot episode as an attendant at a sperm bank, and then made three appearances as an emergency room nurse in "The Peanut Reaction" episode (Season 1) and "The Robotic Manipulation" episode (Season 4), as well as a ward nurse in "The Werewolf Transformation" (Season 5). The name of the character is never used on the show, only on her nametag and in the credits. She is also the only character besides Leonard and Sheldon to be carried over from the original pilot.
Kurt (Brian Patrick Wade): A tall, intimidating bodybuilder, Kurt is Penny's ex-boyfriend at the beginning of the series. In the pilot-episode, Leonard and Sheldon went to his apartment and tried to retrieve Penny's TV set from him, but they did not succeed and returned home without pants. Penny left him because he cheated on her, but he was still invited to her Halloween party, for which he dresses up as a caveman. In season 2, ep14, Penny experiences financial difficulties, Leonard and the guys confront Kurt over $1,800 in legal fine money Penny had lent him. Immediately refusing, Kurt shortly thereafter pays Penny and the two have dinner, but is never seen again.
Summer (Sierra Edwards): Howard's "date" for the reception for Dr. Gablehauser in Season 1, Episode 4, "The Luminous Fish Effect." She is quite a bit taller than Howard and makes it apparent that the "date" is strictly a business event by telling Howard that "touching is extra".
Patty (Rachel Cannon): A friend of Penny, who attends Penny's Halloween party in Season 1, Episode 6, "The Middle Earth Paradigm." Patty is the blonde dressed in 1960's era attire, who first asks Sheldon what his costume is and has little patience for Sheldon's explanatory clues. She is not identified within the episode, but is listed in the credits.
Cheryl (Erin Allin O'Reilly): Apparently a friend of one of Penny's friends (because it is subtly suggested she may not know many of the people at the party), who attends Penny's Halloween party in Season 1, Episode 6, "The Middle Earth Paradigm." Cheryl is the very talkative, short-haired brunette dressed as a ladybug, who hits it off with Raj. She is not identified within the episode, but is listed in the credits.
Vicki (Kimberly Kevon Williams): A friend of Penny, who attends Penny's Halloween party in Season 1, Episode 6, "The Middle Earth Paradigm." Vicki is the black woman dressed as a princess (?) who mistakes the Doppler Effect as a learning disability. She is not identified within the episode, but is listed in the credits.
Roberta (Cynthia Holloway): A friend of Penny, who attends Penny's Halloween party in Season 1, Episode 6, "The Middle Earth Paradigm." Roberta is the dark brunette dressed as a sexy nurse, who is hit on by Howard. She is not identified within the episode, but is listed in the credits.
Chen (James Hong): The owner of a Chinese restaurant the guys frequently visit.
Christy (Brooke D'Orsay): An acquaintance of Penny's from Nebraska who had slept with enough of her relatives to make her "family", Christy suddenly moves in with Penny when she comes to California. Being promiscuous, Christy ends up sleeping with Howard, forcing Penny to sleep on Leonard's couch. Howard then invites Christy to move in with him (and his mother). Mrs. Wolowitz and Christy end up having a fight, and the latter leaves. She also presents the guys other problems by reducing their foursome to three when playing Halo and dividing up their Chinese food order.
Dmitri (Adam Gregor) He is the janitor of third floor in Caltech. But before he was a janitor, he was a physicist in Leningrad Polytechnical. After the collapse of the Soviet Union, he lost his job and had to move to the US. He seems to know a lot about physics. A question in the Physics Bowl about quantum electrodynamics stumped everyone but him.
Lalita Gupta (Sarayu Rao): A childhood acquaintance of Raj who bullied him, she is now a dental student at USC. Raj's parents set up a date for their son with her. During the date, Raj can only speak to her after drinking an alcoholic beverage (a "grasshopper") and acts very obnoxious, such as commenting on her childhood weight problem. Sheldon insists she bears a remarkable resemblance to Princess Panchali, an Indian princess in a children's story. Lalita is flattered at Sheldon's comments and attention. Eventually, Lalita leaves the obnoxious and drunk Raj to have dinner with Sheldon. Sheldon does not see her again because he "already has a dentist".
Toby Loobenfeld (DJ Qualls): A research assistant with a double major in Physics and Theater, Toby was used by Sheldon to play his fictional first cousin "Leopold Houston" from Denton, Texas, a character Sheldon invents as part of an elaborate lie told to avoid going to see Penny sing. Cousin "Leo" is a recovering drug addict who ends up cuddling with Penny on the couch, much to the chagrin of Leonard. He was also willing to argue with Sheldon on the subjects regarding drug use, showing knowledge on the subject.
Dennis Kim (Austin Lee): Dennis is a 15 year old child prodigy from North Korea whom Dr. Gablehauser hopes to recruit into their Ph.D. program. Dennis and Sheldon have an antagonistic relationship when the former excels Sheldon in every way. The guys make arrangements so Dennis can meet girls of his age and get distracted from his own research. The plan succeeds and Dennis is seen kissing with a girl in the park, no longer an intellectual threat to Sheldon.
Missy Cooper (Courtney Henggeler): Missy is Sheldon's twin sister who is intellectually and socially very much unlike him. Confident, tall and attractive, she immediately 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 elects himself as a guardian to Missy in order to choose the best mate for her (she rejects Leonard and Howard in favor of Raj, but his mutism derails any chance he might have with her). Missy objects to Sheldon deciding who she can sleep with, refers to Sheldon as "Shelly", and despite their differences she loves him and is proud of his accomplishments.
Blain (Tyler Olson) and Tom (Mark Hames): A couple of guys Sheldon tries to fix up with Penny in order to help break her video game addiction. In his clumsiness, Sheldon gives Blain the incorrect impression that he is inviting him on a date (with sex) rather than fixing him up on a date with a woman (Penny). Tom is a person whom Sheldon finds in an online dating service who was matched to Penny because he likes the woman to be the sexual seducer.
Ramona Nowitzki (Riki Lindhome) and Kathy O'Brian (Emily Happe): A couple of graduate students, who both have a crush on Sheldon, and become his 'groupies.' Sheldon exploits them to get free food and perform personal services. Ramona is the first, first meeting Sheldon after he introduced his work to incoming graduate students. She finds Sheldon's work very stimulating and him very cute to the utter confusion of his friends and arranges to have dinner with him in his apartment. The next few days she becomes a permanent presence in his life, pressuring him to concentrate on his research and avoid distractions such as video games, sci-fi television shows and weekend paintball expeditions. Sheldon gets tired of her but is unable to dissolve their "relationship". Eventually, Sheldon reaches a breakthrough in his work to which Sheldon acknowledges that he could not have completed without her personal and mathematical assistance. Sheldon kicks Ramona out when she asks to share credit for the discovery. Kathy comes into Sheldon's life after he kicked Ramona out.
Dr. David Underhill (Michael Trucco): A MacArthur Genius Grant recipient, David is an experimental physicist like Leonard, who is excited to be working with him. With his leather jacket and handsome looks, Penny is shocked to know that David is a scientist and starts dating him. They break up when she finds out he is married.
Alicia (Valerie Azlynn): A young woman who moves into an apartment above Leonard and Sheldon in the Dead Hooker Juxtaposition. Penny displays a fit of jealousy at this turn of events, thinking that Alicia is supplanting her in the boys' attention. Alicia is an actress, although slightly more successful than Penny, having landed a role on CSI as a (dead) prostitute. Eventually, to Howard's delight, she and Penny get into a catfight over her treatment of the guys. After the catfight, Penny says Alicia is a "dead whore on TV; live one in real life" just before the gang hears Alicia having sex with a CSI producer.
Dr. Crawley (Lewis Black): An entomologist at the University, Dr. Crawley is visited by Sheldon, Howard, and Raj in order to identify the species of a cricket they found. He is agitated at losing his position at the university, his lab, and, as a result of an extended research trip, his wife.
Bethany (Molly Morgan), Sarah (Sarah Buehler), and Skeeter (Andy Mackenzie): Characters who appear in "The Gothowitz Deviation" episode. Bethany and Sarah are a couple of women that Wolowitz and Raj meet in a Goth club. They then go to a tattoo parlor, where Skeeter is the tattoo artist. The name "Skeeter" is not used on the show, but appears in the credits.
"Captain Sweatpants" (Ian Scott Rudolph) and "Lonely Larry" (Owen Thayer): Two nerds that 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.
Abby (Danica McKellar) and Martha (Jen Drohan): With Leonard and Howard busy on a double date with Penny and Bernadette, Raj and Sheldon attend a university mixer where they meet Abby and Martha. Abby takes a liking to Raj, while Martha seems to connect with Sheldon. While Raj and Abby end up kissing, Sheldon completely ignores Martha, who even tries to go to bed with him. Like series regular Mayim Bialik, McKellar exists within the Big Bang Theory universe independently of Abby, having been mentioned alongside Bialik in ep. 1.13, "The Bat Jar Conjecture".
Venkatesh Koothrappali (Frank Maharajh): Raj's cousin, a lawyer in India, who tries to negotiate a deal for Raj's part of the One Ring prop from Lord of the Rings. He, by his own admission, is a bad negotiator, and does not get Raj the two jet skis that he wanted.
Dr. Elizabeth Plimpton (Judy Greer): A renowned cosmological physicist from Princeton University known for her work in quantum cosmology who comes as a personal guest of Sheldon in "The Plimpton Stimulation" episode. She first appears absent-minded, but then exhibits a strong sexual libido, winds up having sex with Leonard and Raj, and expresses a desire for a foursome with Leonard, Raj, and Wolowitz.
Joyce Kim (Ally Maki): A previously unseen character who appears in "The Staircase Implementation", Joyce Kim was mentioned as a former girlfriend of Leonard; their relationship only lasted a month, after which she defected back to North Korea. However, in The Staircase Implementation, Leonard admits that she was not a girlfriend, but a North Korean spy who attempted to get secrets from him by seducing him.
Joy (Charlotte Newhouse): An exercise-freak woman whom Leonard meets on a blind date set up by Bernadette via the "Girlfriend Pact" with Howard. Although Leonard finds Joy's personality to be repulsive, he agrees to go out with her again based on hints she gives that it will be easy to have sex with her.
Special Agent Angela Paige (Eliza Dushku): an FBI agent investigating Howard Wolowitz's background for a security clearance in "The Apology Insufficiency" episode. While interviewing the guys, Raj vomits on her shoes, Leonard hits on her, and Sheldon reveals too much information about Howard, and to add insult to injury reveals too much information about Leonard.
Wyatt (Keith Carradine): Penny's father. He comes to visit and Penny feels compelled to involve Leonard in a ruse to make him think that 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 grandkids to "grow up in a house without wheels on it"). Strangely though in the first episode where Leonard's mother visits, Penny tearfully reveals her rough childhood with an imitation of what she wanted her mother to say to her father, being "Bob, get over it, she's a girl, move on, but she didn't not one word!".
Professor Glenn (Rick Fox): A former professor of Bernadette's, whom she reveals to Howard that she dated for a year. At 6'7", dark and handsome, Glenn is physically the polar opposite of Howard, leading the insecure Howard to feel like he'll never measure up (literally as much as metaphorically).
President Siebert (Joshua Malina): The president of the California Institute of Technology; he is married and has a teenage daughter. Siebert sends Sheldon on an expedition to the North Pole at the end of Season 2. In Season 4, he organizes a get-together with wealthy donors and faculty in order to garner funds for the university's programs. He urges the guys to make an appearance to schmooze the donors so that they will give to their projects. Seeing Sheldon's behavior toward the donors, he asks that he not appear again. A recurring gag is that President Siebert bumps into Sheldon when he is using the urinal.
Mrs. Latham (Jessica Walter): A wealthy widowed benefactor who takes a shine to Leonard at a donor/faculty get-together. She asks him out with the implication that he have sex with her in order to get the lab equipment his department needs. He at first resists, but after she says that she is going to fund his project anyway, he relents. Everyone at the University congratulates him for selling himself out for the money.
Todd Zarnecki (Christopher Douglas Reed): Appears in "The Zarnecki Incursion" episode as a hacker who "steals" Sheldon's possessions in the World of Warcraft online game. Being physically much larger and more intimidating than the four main characters, they back down from confronting him. Penny picks them up, takes them back to Todd's house and sets Todd straight with a swift kick to the groin. He showed up at Howard's bachelor party.
Emily (Katie Leclerc): A deaf woman whom Penny plays matchmaker for Raj. Since she is deaf, Raj finds that he can communicate with her despite his selective mutism. She shows interest in Raj when he spends a lot of money on her. Then Raj's parents get the bill, and force him to break up with her. Although Raj resists, he does tell Emily that all the gifts he gave her have to go back. She then loses interest in Raj.
Kevin (Blake Berris): A man whom Penny meets during her 'non-date' with Leonard. He is writing a screenplay about his roommate who threw him out of their apartment for the evening.
Laura (Ashley Austin Morris): A woman whom Leonard chats up during his 'non-date' with Penny after Penny starts talking to Kevin. Leonard can't focus on their conversation for watching Penny and Kevin, and he can't remember her name.
Dale (Josh Brener): A young man that Leonard and Sheldon meet when they are arrive at the comic book store and are surprised to see working the register instead of Stewart. When asked about Stewart's whereabouts, Dale responds that he was put in charge while "he went out for coffee, with a girl" (who happens to be Amy Farah Fowler), and states he doesn't get why Stuart put him in charge after Leonard questions him.
Alice (Courtney Ford): An attractive comic book enthusiast who meets Leonard at the comic book store, who thinks he's cute and then has him to her apartment and tries to hook up with him. This date occurs during the period when Leonard was having a long distance relationship with Priya. Leonard initially gives in to temptation, but at the last minute pulls away. He tells Alice about her, that his super power is being a good guy and gets thrown out of her apartment. Next time he's communicating, via the webcam, with Priya, he confesses his weakness. To his surprise, Priya has no problem with it. Upon questioning, Priya reveals that she has not been faithful to Leonard. They then break up.
Jimmy Speckerman (Lance Barber): a former high-school jock who used to bully Leonard on a regular basis. He later contacts Leonard in "The Speckerman Recurrence", with an idea of inventing glasses that turn all movies into 3D. He is oblivious to the bullying nature of his actions against Leonard, having believed that they were "having fun" and describing him and Leonard as having been "a comedy team".
Siri (Becky O'Donohue): embodies Siri (software), the voice-recognition system used on certain smartphones and on which Raj developed a crush, because of its sexy voice. To which Howard jokingly states that "there is finally a woman in your life you can talk to." Raj has a dream about meeting her, represented by a stunningly beautiful woman, but he cannot speak to her in person due to his selective mutism, which causes him to awake from the dream screaming "NOOOOO!".
Spock (Leonard Nimoy): Sheldon's action figure Spock, the voice of Leonard Nimoy, appears to him in a dream convincing him to open his mint in-box 1975 Star Trek Transporter toy. After Sheldon breaks his toy and steals Leonard's, Spock reappears to Sheldon and convinces him to give Leonard his toy back.
Lakshmi (Chriselle Almeida): a woman with whom Raj's parents set him up when he asks for a date to take to his cousin's wedding. He develops strong feelings for her and is willing to marry her, and she him. She tells him she is a lesbian and is willing to marry him because she thinks he is gay and the ruse will fool her parents.
Mr. Rostenkowski (Casey Sander): Bernadette's father, a retired police officer. He has a rough and rude personality, and is not very accepting of Howard until he learns that Howard will be going into space as an astronaut.
Cole (Ryan Cartwright) : A British student who works with Penny in the history class in Pasadena City College, introduced in the episode The 43 Peculiarity (S6E08). His appearance in Penny's life leads to Leonard's anxiety about his relationship with Penny.
Mrs. Rostenkowski (Meagen Fay) : Bernadette's mother. She is a woman of few words, like her husband. She displays a lot of the same characteristics of her daughter. Apparently she dominates her husband.
Mrs. Jeanine Davis (Regina King): A human resources representative working at the university who handled a sexual harassment complaint against Sheldon. Sheldon, in his attempt at defense, rats out "worse behavior" by Raj, Howard, and Leonard, prompting Mrs. Davis to summon them to her office as well. In a later episode, Mrs. Davis 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 "butter up" Mrs. Davis.
Lucy (Kate Micucci): A woman who shows up at the party at the comic book store on Valentine's Day for people who had no dates. She and Raj hook up and subsequently start dating. She has social anxiety issues, which allows her and Raj to connect. On their date, she texts with Raj and, at the end, she offers to kiss him, but chickens out at the last second.
Arthur Jeffries (aka Professor Proton) (Bob Newhart): The star of the science show which Sheldon and Leonard (and probably Howard) watched as children. After the show was cancelled, Professor Proton was not taken seriously as a scientist and resorted to doing children's parties as his persona. Sheldon hires him to do a private party for Leonard and himself (and Penny) and he ends up having a heart attack. He asks Sheldon to do a party for a Korean family, which Sheldon agrees to do as Professor Proton, Jr.
Characters that appeared in only a single scene 
These characters appear in a single scene. The list is sorted by chronological order of appearance, considering their first appearance in the show.
Octavia (Octavia Spencer): a clerk working at the California Department of Motor Vehicles who hands Sheldon a drivers permit after he refuses to take the test, but instead points out inaccuracies in the test. The name of the character is not used on the show, only in the credits.
Sandy (Yeardley Smith): A bureaucrat who interviewed Sheldon for a menial job in The Einstein Approximation. episode. Smith had a recurring role on Dharma and Greg as Marlene (both characters have similarities) a show previously co-created by Chuck Lorre. The name "Sandy" was not used on the show, but appeared in the credits.
Sebastian (Steven Yeun) Sheldon's former roommate who left Sheldon on bad terms. Sebastian makes a single appearance in "The Staircase Implementation", where he tells Leonard to "run fast, run far" from Sheldon. His name is not used in the episode, but is listed in the credits.
Louis (Ajgie Kirkland) A former occupant of the apartment in which Penny lives. He is a cross dresser whom Leonard mistakenly runs into in "The Staircase Implementation" episode, and who refers to Sheldon as "the crazy guy across the hall". The name Louis is used by Sheldon, referring to him/her as Louis/Louise. Sheldon implies that Louis is a cop.
Mrs. Fowler (Annie O'Donnell): Amy Farrah Fowler's mother. She appeared in a webcam conversation with Amy and Sheldon in which they lie to her about the nature of their relationship, telling her that they are in a romantic relationship that includes coitus.
Angelo (Peter Onorati): A barber who substitutes for Sheldon's regular barber, Mr. D'Onofrio, when Mr. D'Onofrio falls into a coma.
Mrs. Gunderson (Lauri Johnson): Leonard and Sheldon's downstairs neighbor, she appeared briefly in season 3 episode 23 ("The Lunar Excitation") when Leonard was arguing with Penny in the stairwell.
Notable guest stars appearing as themselves 
Summer Glau (Season 2, "The Terminator Decoupling"): When the guys travel by train to a conference in San Francisco, they realize Summer Glau (who was in two science fiction television shows, Firefly and Terminator: The Sarah Connor Chronicles) is sitting in the same passenger car. Raj, Howard, and Leonard take turns talking with her. Raj cannot talk without drinking beer (which turned out to be non-alcoholic), Howard is his usual creepy, over-the-top self, and Leonard cannot start a conversation before she has to get off the train.
Katee Sackhoff (Season 3, "The Vengeance Formulation" and Season 4, "The Hot Troll Deviation"): Howard fantasizes about taking a bath with Katee Sackhoff (Captain Kara "Starbuck" Thrace in Battlestar Galactica). However, in his own fantasy Sackhoff scolds him and tells him to get a real girlfriend instead of imaginary ones. Sackhoff reappeared in a different fantasy of Howard's in The Hot Troll Deviation, where she and George Takei help Howard realize that he still has feelings for Bernadette.
George Takei (Season 4, "The Hot Troll Deviation"): During one of Howard's fantasies, Takei, along with Katee Sackhoff, help Howard realize that he still has feelings for Bernadette. Takei's real life homosexuality is twice referenced in the episodes: once, when he first appears in Howard's fantasy, Sackhoff asks Howard if Takei's presence implies that Howard has homosexual tendencies, and the second reference occurs when Takei advises Howard about women's love preferences. Sackhoff asks Takei "How would you know?" and he responds "I read."
Neil deGrasse Tyson (Season 4, "The Apology Insufficiency") Appears as a colleague of Raj. Upon being introduced to Tyson, Sheldon tells him that he (Sheldon) is upset at Tyson's role in the demotion of Pluto from planet status. Initially, Tyson explains that he had no role in the demotion, but later attempts to apologize to Sheldon. Sheldon declines the apology, immediately after he himself apologized to Howard, who declined Sheldon's apology.
Stephen Hawking (Season 5, "The Hawking Excitation" and Season 6, "The Extract Obliteration"): After Sheldon's much-anticipated meeting with Hawking, he discusses Sheldon's research and points out a mistake. Sheldon protests by saying that he doesn't make mistakes, to which Hawking replies "Are you saying that I do?" When Sheldon realizes his research does in fact contain a math error, he faints in embarrassment, prompting Hawking to remark "Oh great, another fainter."
Mike Massimino (Season 5, "The Friendship Contraction","The Countdown Reflection",Season 6, "The Decoupling Fluctuation and "The Re-Entry Minimization"): An astronaut who appears as a future colleague of Howard who is set to join NASA's astronaut corps. Massimino reveals that his nickname is "Mass". Howard assumes the nickname is derived from Newton's second law, which states that force = mass times acceleration, but Massimino reveals that "Mass" is simply short for Massimino. Raj and Howard concoct an elaborate scheme to plant a seed in Massimino's mind to give Howard the nickname "Rocket Man" by having Howard set up the song "Rocket Man" as his ringtone on his cell phone, and Raj calling the cell phone during Howard's Skype conversation with Massimino. Their plan is thwarted when Howard's mother shouts to him that he needs to finish his Froot Loops, which triggers Massimino to give Howard the nickname "Froot Loops". He is heard on the phone in "The Launch Acceleration" and in the Soyuz capsule with Howard and cosmonaut Dimitri Rezinov, serving as their mission commander in "The Countdown Reflection".
Cameo appearances 
Charlie Sheen (Season 2, "The Griffin Equivalency"): When Raj discovers a celestial body he is very excited about it. At Penny's restaurant, Raj says to the patron at the next table over, "Hey, buddy. I'm going to be in People magazine!"; Charlie Sheen turns around, takes his sunglasses off and says "Yeah? Call me when you're on the cover." (At the time, Sheen was starring in Two and a Half Men, also produced by Chuck Lorre.)
Analeigh Tipton and Samantha Potter (Season 2, "The Panty Piñata Polarization"): Howard and Raj visit the America's Next Top Model house pretending to be cable television repairmen. Tipton and Potter made appearances, opening the door for Howard and Raj. Tipton greeted Howard and Raj, caught them in a lie as the house does not receive cable television (it receives satellite), but allows them in regardless when Howard "corrects" his error with another lie when he says that they are satellite television repairmen. Potter did not get a speaking role.
George Smoot (Season 2, "The Terminator Decoupling"): After a train ride to a conference in San Francisco, Sheldon presents his paper to 2006 Nobel Prize laureate George Smoot, and proposes joint research; Smoot abruptly rejects his idea by asking "With all due respect, Dr. Cooper, are you on crack?"
Ira Flatow (Season 3, "The Vengeance Formulation"): Sheldon is invited to talk about magnetic monopoles on Flatow's radio show, Science Friday. However, Barry Kripke pulls a prank on Sheldon, and Sheldon is publicly humiliated to a nationwide audience. Flatow's appearance was voice only.
Stan Lee (Season 3, "The Excelsior Acquisition"): When Sheldon has to miss Lee's appearance at the comic book store because he ends up in traffic court after running a red light, Penny tries to make it up to him by taking him to Lee's house, uninvited and unannounced (acquiring Lee's address from Stuart). Lee, not impressed by the unexpected intrusion, sarcastically remarks "why don't you just come in and watch the Lakers game with me?" only for Sheldon, not understanding sarcasm, to rush into the house. Lee, annoyed, decides to sue for a restraining order. When Sheldon announces his "autographed" restraining order to Leonard, Howard, and Raj, he comments it will look great hanging next to his restraining order from Leonard Nimoy.
Steve Wozniak (Season 4, "The Cruciferous Vegetable Amplification") Dines in Penny's restaurant at the same time that the guys are there. He compliments Sheldon for his "virtual presence device" (using Sheldon's name for the device without having heard how Sheldon calls it) and in turn Sheldon (having previously referred to him as "the Great and Powerful Woz") compliments Wozniak telling him that he ranks 15th on his list of favorite technological visionaries. When Wozniak feigns disappointment, Sheldon tries to console him by stating that the ranking is six places ahead of Apple founder Steve Jobs. Sheldon then tells Wozniak that the Apple II was rather "nifty" and is one of his proudest possessions. Wozniak promises Sheldon that he would autograph the computer if Sheldon brought it to him. In the haste to get the Apple II to Wozniak for signing, Sheldon trips and falls down the stairs, breaking the computer.
LeVar Burton (Season 4, "The Toast Derivation" and Season 6, "The Habitation Configuration"): Sheldon invited Burton to a party via a tweet. Upon opening the door and seeing the other guests at the party, Zack, Kripke, and Stuart (who at that moment was wearing nothing but a towel) singing karaoke, Burton leaves and when starting to walk down the apartment block stairs utters that he is "so done with Twitter". He appears again at the end of "The Habitation Configuration", taping an episode of "Fun With Flags" with Sheldon and Amy. Amy says "cut", followed by complaining that "this guy is worse than Wil Wheaton" (referring to Wil's appearance earlier in the episode). Sheldon then tells Burton that because Amy is his girlfriend, he is obligated to agree with her. Burton asks "I still get lunch, right?", to which Sheldon nods. Along with Brent Spiner and Wil Wheaton, Burton makes Star Trek: The Next Generation the most represented franchise on the show in terms of guest appearances.
Brian Greene (Season 4, "The Herb Garden Germination"): Sheldon and Amy attend a book signing event of Greene's popular science book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos. At the event, Sheldon initially ridicules Greene in private conversations with Amy, but then he gets up and publicly ridicules Greene, asking Greene why doesn't he do something more useful with his time than write popular science (only to turn around and state he was kidding).
Brent Spiner (Season 5, "The Russian Rocket Reaction"): Sheldon shows up at a party thrown by his "mortal enemy", Wil Wheaton, only after he hears that Spiner will be there. After Wheaton gives Sheldon a signed, in-the-box Wesley Crusher action figure, Spiner grabs it and opens it, thus putting him on Sheldon's Mortal Enemies List immediately after Wheaton was removed from the list. Along with LeVar Burton and Wil Wheaton, Spiner makes Star Trek: The Next Generation the most represented franchise on the show in terms of guest appearances.
Howie Mandel (Season 6, "The Re- Entry Minimization"): When Howard returns to earth he hears shouts of "Howie" and believes that the paparazzi is here for him until he learns that they were there for Mandel who had arrived at the airport at the same time as Howard. Mandel thinks Howard is delusional, proclaiming to his driver that during the entire flight, "he kept telling me he's an astronaut."
Buzz Aldrin (Season 6, "The Holographic Excitation"): Raj sends Howard a link showing a video that features Buzz Aldrin handing out candy to kids on Halloween. Aldrin gives space related candy, such as Milky Way, Mars bar, and moonpies. He brags to the kids about his amazing achievement of being an astronaut who walked on the Moon, then taunts them with the line, "what have you done with your life?"
- "The Middle Earth Paradigm". The Big Bang Theory. Season 1. Episode 6. October 29, 2007. 5:18 minutes in.
- "The Euclid Alternative". The Big Bang Theory. Season 2. Episode 5. October 20, 2008. CBS.
- "The Vegas Renormalization". The Big Bang Theory. Season 2. Episode 21. April 27, 2009. 9:42 minutes in.
- "The Luminous Fish Effect". The Big Bang Theory. Season 1. Episode 4. October 15, 2007.
- "The Guitarist Amplification". The Big Bang Theory. Season 3. Episode 7. November 9, 2009. CBS.
- "The Electric Can Opener Fluctuation". The Big Bang Theory. Season 3. Episode 1 (41). September 21, 2009. CBS.
- "The Apology Insufficiency". The Big Bang Theory. Season 4. Episode 7. November 4, 2010. 12:50 minutes in.
- "The Countdown Reflection". The Big Bang Theory. Season 5. Episode 24. May 10, 2012.
- "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 Cruciferous Vegetable Amplification". The Big Bang Theory. Season 4. Episode 2. September 30, 2010.
- "The Middle Earth Paradigm". The Big Bang Theory. Season 1. Episode 6. October 29, 2007.
- Season 3 Episode 23 The Lunar Excitation
- "The Roommate Transmogrification". The Big Bang Theory. Season 4. Episode 24. May 19, 2011.
- "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 Creepy Candy Coating Corollary". The Big Bang Theory. Season 3. Episode 5. October 19, 2009.
- "The Vengeance Formulation". The Big Bang Theory. Season 3. Episode 9. November 23, 2009.
- "The Holographic Excitation". The Big Bang Theory. Season 6. Episode 5. October 25, 2012.
- "The Habitation Configuration". The Big Bang Theory. Season 6. Episode 7. November 8, 2012.
- "The Cooper/Kripke Inversion". The Big Bang Theory. Season 6. Episode 14. nuary 31, 2013.
- "The Zazzy Substitution". The Big Bang Theory. Season 4. Episode 3. October 7, 2010. 5:53 minutes in.
- "The Flaming Spittoon Acquisition". The Big Bang Theory. Season 5. Episode 10. November 17, 2011. Event occurs at 11:44. CBS. Sheldon is seen looking at Stuart's Facebook page, where his name is seen to be "Stuart Bloom"
- "Big Bang Theory Makes 'Stuart' a Series Regular". TVLine.com. Retrieved August 22, 2012.
- "The Hofstadter Isotope". The Big Bang Theory. Season 2. Episode 20. April 13, 2009.
- "The Classified Materials Turbulence". The Big Bang Theory. Season 2. Episode 22. May 4, 2009.
- Sleasman, MaryAnn. The Big Bang Theory S6E13 blog at TV.com
- "The Jerusalem Duality". The Big Bang Theory. Season 1. Episode 12. April 14, 2008.
- "The Bat Jar Conjecture". The Big Bang Theory. Season 1. Episode 13. April 21, 2008.
- "The Panty Piñata Polarization". The Big Bang Theory. Season 2. Episode 7. November 10, 2008.
- "The Cornhusker Vortex". The Big Bang Theory. Season 3. Episode 6. November 2, 2009.
- "The Griffin Equivalency". The Big Bang Theory. Season 2. Episode 4. October 13, 2008.
- "The Lizard-Spock Expansion". The Big Bang Theory. Season 2. Episode 8. November 17, 2008.
- "The White Asparagus Triangulation". The Big Bang Theory. Season 2. Episode 9. November 24, 2008.
- "The Vartabedian Conundrum". The Big Bang Theory. Season 2. Episode 10. December 8, 2008.
- "Big Bang Theory: We didn't anticipate how protective the audience would feel about our guys". Variety. May 5, 2009. Retrieved March 7, 2010.
- "The Maternal Capacitance". The Big Bang Theory. Season 2. Episode 15. February 9, 2009.
- "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 Toast Derivation". The Big Bang Theory. Season 4. Episode 17 (80). February 24, 2011.
- "The Lunar Excitation". The Big Bang Theory. Season 3. Episode 23. May 24, 2010.
- "The Irish Pub Formulation". The Big Bang Theory. Season 4. Episode 6. October 28, 2010. CBS.
- The Infestation Hypothesis
- "The Good Guy Fluctuation"
- Sleasman, MaryAnn. The Big Bang Theory S6E13 blog at Tv.com
- "Pilot". The Big Bang Theory. Season 1. Episode 1. September 24, 2007.
- "The Financial Permeability". The Big Bang Theory. Season 2. Episode 14. February 2, 2009.
- "The Dumpling Paradox". The Big Bang Theory. Season 1. Episode 7. November 5, 2007.
- "The Loobenfeld Decay". The Big Bang Theory. Season 1. Episode 10. March 24, 2008.
- "The Pork Chop Indeterminacy". The Big Bang Theory. Season 1. Episode 15. May 5, 2008.
- "The Cooper-Nowitzki Theorem". The Big Bang Theory. Season 2. Episode 6. November 3, 2008.
- "The Bath Item Gift Hypothesis". The Big Bang Theory. Season 2. Episode 11. December 15, 2008.
- "The Dead Hooker Juxtaposition". The Big Bang Theory. Season 2. Episode 19. March 30, 2009.
- "The Jiminy Conjecture". The Big Bang Theory. Season 3. Episode 2. September 28, 2009.
- "The Einstein Approximation". The Big Bang Theory. Season 3. Episode 14. February 1, 2010.
- "(#402) "The Cruciferous Vegetable Amplification"". the Futon Critic. Retrieved September 9, 2010.