A breakout character is a character in serial fiction who becomes a more prominent, popular, discussed, and/or imitated individual. The series from which the breakout character springs may be in the form of a novel, magazine, television series, comic strip, game or combination.
Bullwinkle J. Moose was voiced by Bill Scott on Rocky and His Friends. Although the series was originally named after Rocky the Flying Squirrel, it was Rocky's dim-witted (and much larger) sidekick, Bullwinkle J. Moose, who got most of the jokes while Rocky served as straight man. By 1961, the series had been renamed The Bullwinkle Show, a title that appears on syndicated reruns of the series to this day.
Krazy Kat evolved from an earlier comic strip of series artist George Herriman's, The Dingbat Family, which started in 1910 and was later renamed The Family Upstairs. This comic chronicled the Dingbats' attempts to avoid the mischief of the mysterious unseen family living in the apartment above theirs and to unmask that family. Herriman would complete the cartoons about the Dingbats, and finding himself with time left over in his 8-hour work day, filled the bottom of the strip with slapstick drawings of the upstairs family's mouse preying upon the Dingbats' cat. This "basement strip" grew into something much larger than the original cartoon. It became a daily comic strip with a title (running vertically down the side of the page) on October 28, 1913 and a black and white full-page Sunday cartoon on April 23, 1916. Due to the objections of editors, who considered it unsuitable for the comics sections, Krazy Kat originally appeared in the Hearst papers' art and drama sections. Hearst himself, however, enjoyed the strip so much that he gave Herriman a lifetime contract and guaranteed the cartoonist complete creative freedom.
Nero from The Adventures of Nero by Marc Sleen was originally introduced as a side character in the series De Avonturen van Detective Van Zwam, where Detective Van Zwam was the main protagonist. From the first Van Zwam story on, Het Geheim van Matsuoka ("Matsuoka's Secret") (1947) readers reacted more enthusiastically to the dumb, lazy, vain and stubborn character Nero than the more noble and clever Van Zwam. So, from "De Hoed van Geeraard de Duivel" ("The Hat Of Gerard the Devil" (1950)) onward the series was named after Nero instead.
Popeye first appeared 10 years into the run of Thimble Theatre, a comic strip started in 1919 by E.C. "Elzie" Segar for the King Features Syndicate. The strip initially focused on Olive Oyl, her family, and her boyfriend Ham Gravy. Segar introduced Popeye as a sailor hired by Olive's brother Castor to facilitate a single adventure. When the sailor character disappeared from the strip afterwards, fans demanded his return, and the series later became Thimble Theater, Starring Popeye.
The Smurfs were originally supporting characters in Peyo's comic series Johan and Peewit in 1958. The massive popularity of the little blue men led to them getting their own series a year later, which was subsequently followed by massive merchandising, a television series and various other productions.
Throckmorton Philharmonic Gildersleeve (as portrayed by Harold Peary) was an antagonist on the long-running radio comedy Fibber McGee & Molly. The underwear salesman proved popular enough to warrant a spin-off, The Great Gildersleeve, which, like its parent show, would go on to a long run in radio, film and (briefly) television.
Kerr Avon, portrayed by Paul Darrow in "Blakes 7", was introduced in the 2nd episode, Space Fall. His character quickly became a major hit with fans due to his sardonic humour and distrust of emotions, eventually replacing Roj Blake, portrayed by Gareth Thomas, as series lead at the end of the 2nd series, and going on to feature in his own trilogy of novels set after the end of the series.
Carson Beckett, portrayed by Paul McGillion on Stargate Atlantis, was introduced in the pilot episode, originally intended to be an occasional guest star for scenes requiring a doctor. His character was an immediate hit with the fans from inception, and Beckett earned his own episode halfway through Season 1. He was upgraded to a regular in Season 2, becoming one of the six main characters and appearing in 15 episodes of the season. Despite his popularity, however, the character was killed off at the end of Season 3, which led to outrage among his fans, who campaigned so heavily for his return that the character was written back into the series a year later. He became a recurring character once again during the show's fourth and fifth seasons.
Castiel (Misha Collins) on Supernatural is noted for originally being conceived for a short six-episode story arc at the beginning of the show's fourth season. By the time the fourth season came to a close, not only had the character quickly become a favorite amongst fans, but he was subsequently upgraded from his previous supporting status to a series star alongside the show's main protagonists Sam Winchester and Dean Winchester from the show's fifth season onward.
Oswald Cobblepot (Robin Lord Taylor) in Gotham. He has been called the show's breakout character. This version of the character is a ruthless small-time criminal working for mobster Fish Mooney (Jada Pinkett Smith).
Chad Dylan Cooper (Sterling Knight). He is the teen heart-throb of Tween Weekly and Mackenzie Falls, and the main antagonist in season one. He is the arch-rival of Sonny and the cast of So Random! early in the series, as well as Sonny's love/hate interest as developed later.
Sheldon Cooper (Jim Parsons) on The Big Bang Theory. Sheldon was supposed to be a sidekick of the character of Leonard, but he became so popular that not only did he become the main character of the series but he was often referred to as being the main reason for the show's huge success. Sheldon has been considered by critics to be one of the best TV characters of all time, earning Jim Parsons four Emmys.
Elmo (voiced and operated by numerous puppeteers, but primarily by Kevin Clash from 1984 to 2012) on Sesame Street, he joined the cast of the children's show in the late 1970s. Originally a supporting character, Elmo's popularity among the show's younger fans rose in the 1990s, which led to him receiving his own segment within the show, "Elmo's World", and becoming a major marketing icon.
J.J. Evans (Jimmie Walker) in Good Times, with his catch phrase "Dy-no-mite!", came to dominate the family series, leading to friction with stars Esther Rolle and John Amos, who played his parents. Amos and Rolle's concern was not so much that they resented being upstaged, but because they felt the J.J. character was too stereotypical and not a good role model for young African American viewers. A showdown with the show's producers in 1976 led to modification of the character, Amos's character being killed off and a temporary departure by Rolle from the show. Rolle returned at the beginning of the show's final season in 1978–79, and J.J. became an even stronger focus of the show.
J.R. Ewing (Larry Hagman) in Dallas. The initial concept of Dallas was a Romeo and Juliet-esque tale, focusing on two star-crossed lovers whose families are sworn enemies, with the amoral brother J.R. serving as a supporting character. However, the popularity of J.R. (and Hagman in the role) grew, and the producers acknowledged his new status as the series' breakout character. Two highly rated 1980 episodes became pop culture zeniths. In "A House Divided" and "Who Done It?", the audience witnessed J.R. being shot by an unknown assailant. After the cliffhanger was broadcast in March 1980, the audience was forced to wait until the October premiere of the next season for the cliffhanger's resolution. The summer of 1980 saw the emergence of a national obsession known as "Who shot J.R.?". Riding the crest of his new-found popularity, Larry Hagman threatened to leave the series unless his contractual demands were met. CBS leaked rumors of recasting, but Hagman eventually prevailed. As the series progressed, J.R. emerged as the central character until the show's cancellation in 1991, with Hagman serving as executive producer for the final few seasons. Hagman would go on to reprise the character in two TV movies and a revival series until Hagman's death in 2012.
Arthur Fonzarelli (Henry Winkler) in the AmericansitcomHappy Days began as a fringe character, but quickly evolved into the focal point of the series. His character became the best friend to the main character, Richie Cunningham, displacing Potsie Weber's status as best friend. Winkler's billing in the credits rose to second (he refused to appear above Ron Howard, the star) and then first after Howard left the show to pursue directing. At one point, network executives hoped to retitle the show Fonzie's Happy Days.
Mellie Grant portrayed by Bellamy Young in Scandal. Originally a recurring character meant to appear in only three episodes of the first season, the role of Mellie ended up appearing in every episode, became a main cast member by the second season, and by third was described by many as the breakout character of the show. Praised from the start as a villainous scene stealer, Mellie eventually became much more developed and eventually integral to show, with Young receiving major acclaim for her performance, with one critic going so far as to say, "In Mellie, the show has its most fleshed-out character and in Young, its most compelling performer." 
Super Hans, portrayed by Matt King in Peep Show, was introduced as a minor character in the first series, but proved to a hit with fans.
Jim Ignatowski (Christopher Lloyd) on Taxi was originally a one-time character, a reverend who presided over Latka Gravas' wedding. The next season he was reintroduced, and later in that season he was added to the main cast. The character remained a regular until the show ended in 1983.
Benjamin Linus (Michael Emerson) in Lost was originally only supposed to be in three episodes of Season 2 in the fake persona of "Henry Gale", but the producers enjoyed his performance so much that they wrote him in as the leader of the Others. He became a series regular in Season 3 and remained a star character for the rest of the show. During the series' run, Linus was often hailed as one of the best villains on television, and Emerson was nominated for three Emmys, winning one for Outstanding Supporting Actor in a Drama Series.
Todd Manning (originally and currently Roger Howarth, was played at one time by Trevor St. John) on One Life to Live, known for initiating the gang rape of Marty Saybrooke in 1993, was originally supposed to have a short-lived recurrence. However, once Howarth was seen to attract positive viewer reaction, the character was given a more primary focus. The character's popularity continued even after St. John assumed the role in 2003. (Note: St. John's version of the character was eventually rewritten as Todd's twin brother, Victor. Howarth returned as Todd in 2011.)
Elijah Mikaelson (Daniel Gillies) on The Vampire Diaries and The Originals, was originally brought in as a minor threat for the main characters and as a way to introduce villain Klaus. He was then supposed to die after six episodes. However, the character became so immensely popular that the writers changed his storyline, revealing him to be Klaus' brother. He became an important ally to the main characters and continued to recur on the show. Gillies then brought the character over to the spin-off The Originals, where Elijah has become one of the main characters.
Chloe O'Brian (Mary Lynn Rajskub) on 24. Rajskub's character first appearing during the third season of the series and initially appeared as a recurring character throughout seasons three and four before being promoted to a series regular in season five and continued in that role until season eight as well as reprising her role in 24: Live Another Day. By season six, Rajskub had become the second-billed cast member after lead actor Kiefer Sutherland and has second most appearances of any character after Jack Bauer. During her tenure on the series, Chloe becomes one of Jack's closest friends and allies and is considered a "fan-favorite" and has been included in AOL's list of the "100 Most Memorable Female TV Characters".
Si Robertson on Duck Dynasty. Barbara Walters identified him as the breakout character of that reality TV series when interviewing the cast of that show for her Barbara Walters Presents the 10 Most Fascinating People of 2013 TV special.
Felicity Smoak (Emily Bett Rickards) on Arrow was introduced as an IT consultant at the main character's company midway through the first season. She quickly became a fan favorite and was made a series regular for the second season, with DC comics re-imagining her character for the New 52 as a facsimile of the live-action one.
Spike (James Marsters) on Buffy the Vampire Slayer was originally intended to be a villain for a few episodes, but the character became recurring through the end of the second season, then a main character in the fourth season. He appeared regularly through the end of the series, then appeared as a main character during the final season of Angel.
Spock (Leonard Nimoy) on Star Trek was the only character to be carried over from the original pilot to the second. Series creator Gene Roddenberry was pressured by NBC to drop the character from the second pilot, then later to keep the character in the background. Spock's popularity grew, and NBC soon reversed its stance, encouraging more focus on the character. Spock appeared in every episode of the original series, the animated series and the original cast movies.
Julia Sugarbaker (Dixie Carter) in Designing Women. Also known as "the terminator", Julia was the star of a sitcom about four women and one man working together in her interior design firm at her own house in Atlanta during its seven seasons between 1986 and 1993. Carter's character was "the voice of reason" from the group, and was easily the most remarkable because of her speeches and tirades in a very elegant and dignity way, always deffending her opinion and her workmates' behalf. Carter's character, who was a Democrat, had often released liberal speeches during the series, but Carter herself was a Republican, so the actress would received a return for anything "too much liberal" that her character would have to say in the show: A chance to sing in a future episode.
Steve Urkel (Jaleel White) on Family Matters was originally a one-shot character during the show's first season in 1989. He became so popular that he became a regular cast member from season two forward, practically synonymous with the series.
Blair Waldorf (Leighton Meester) in Gossip Girl was the series' most critically acclaimed character, earning mainstream media recognition from Forbes,Rolling Stone,Variety, and numerous other periodicals. The character was acclaimed as having "stolen the spotlight" in the first season. Her wardrobe garnered real-life coverage from fashion outlets, and she has been cited as a trend-setter outside of the show.
^Miller, Ron (2005). "They really were a great bunch of happy people". TheColumnists.com. Archived from the original on July 16, 2011. Retrieved July 11, 2009. Originally, the Arthur "Fonzie" Fonzarelli character was to be a comic relief dropout type, put there for comic contrast to the milquetoast Richie and his pals. He was a tall, lanky guy, but when Henry Winkler blew everybody away at his reading, they decided to cut Fonzie down to Henry's size. Ultimately, Winkler molded the character around himself and everybody, including Ron Howard, realized this would be the show's "breakout" character.
^Hangin' with Mr. Cooper, MSN Movies This MSN review of the DVD set of second-season episodes of Hangin' with Mr. Cooper refers to "Marquise Wilson, a new regular who was evidently intended to be the series 'breakout' character, a la Urkel on Family Matters".