Arthur "The Fonz" Fonzarelli from Happy Days is the Trope Codifier to such a degree that many new or casual viewers (removed from the context of the show's run) won't realize he's an example at all; however, The Fonz is pretty much the yardstick by which this entire trope is measured, he's the main trope image for a reason, and by the same token, the trope itself was originally codified as "The Fonzie" for a reason. Happy Days was originally written to focus on Ron Howard's Richie Cunningham character with Anson William's Potsie as his co-lead. The "Fonz" character was only supposed to be an incidental, recurring background gag character, but his popularity grew so dramatically that, eventually, the network even tried to change the title to Fonzie's Happy Days until Henry Winkler himself (along with the rest of the cast and the director) protested it. Most of the plots did begin to revolve around The Fonz character regardless and the Animated Adaptation did end up being called the The Fonz and the Happy Days Gang.
All My Children: Erica Kane is practically the definition of this trope, played by Susan Lucci, Erica started out as little more than Pine Valley's teenage Lovable Alpha Bitch recurring antagonistic foil to the shows classic soap heroine Tara, but over time her character proved so successful and popular combined with her larger than life personality, she quickly became the shows leading lady and the overall central character, and the only one to last the entire 41 year run, as well as the most iconic and famous character in the history of daytime television.
Gaheris Rhade from Andromeda. Killed off in the pilot, then expanded upon a bit in flashbacks, then we meet his Identical Grandson, Telemachus, then it turns out he tried to restore the Commonwealth in an alternate timeline, and finally, Telemachus joins as a regular.
Are You Being Served?: The original concept was originally to have Mr. Lucas and Miss Brahms as the leads, being the younger and more "normal" characters. However, the more flamboyant Mrs. Slocombe and Mr. Humphries stole the spotlight.
Felicity Smoak was originally supposed to be just a one-episode tech support agent at Queen Consolidated. But then things happened while shooting that episode and the story was modified to include her in the team. By season two, she was a regular and began to replace Laurel Lance as the go-to character for Ship Tease with Oliver. With the third and fourth season, her screen time would give you the impression that she is the lead female on the show.
Sara Lance is initially introduced posthumously in season 1 flashbacks, she was then revealed to be alive in season 2 using the alias Canary. She was essentially introduced to set up Laurel's Character Development into becoming Black Canary, however she quickly became a Fan Favourite character amongst the fandom with many fans believing she should stay the Canary instead. After the hugely negative reception to her unceremonious death in the first episode of Season 3 (which to many fans felt like a combination of Bury your Bisexuals and fridge stuffing she was brought back to life and promoted to main characters status in Legends of Tomorrow as the White Canary.
While everyone's performances are generally well-received, it is universally acknowledged that it was Tom Cavanagh's intriguing and multi-layered performance as the likeable but mysterious (and somewhat sinister) Dr. Harrison Wells that really drove most of the show's first season. In fact, he's so good that the showrunners found a way to bring him back for Season 2 and potentially beyond through his Earth-2 counterpart, and in later seasons as his various counterparts from The Multiverse.
Leonard Snart, who would steal the scene whenever he appeared, and quickly became a fan-favorite. Most agree he was also the main selling point for Season One of Legends of Tomorrow. It really goes to show how popular he is when he kept getting brought back post-mortem in some capacity, and later permanently via Back Up Twin.
Black Siren, the Earth-2 counterpart of Dinah Laurel Lance. Introduced as The Dragon to the Season 2 Big BadHunter Zolomon / Zoom not long after the death of the original Laurel from Earth-1, she quickly became popular with fans. With her being Truer to the Text compared to the original Laurel and her existence alone being seen as a second chance to write the character properlynote Then Arrow showrunners Marc Guggenheim and Wendy Mericle got a lot of scorn from the comic community and a number of fans for sidelining the character who was so important to source material's mythos, even treating her as a Creator's Pest while also having the potential to be a different character in her own right, many rallied for her to be the Arrowverse's official version of the Black Canary. She's eventually transplanted to Arrow (sort of, as said Earth-1 Laurel was an Arrow main character, meaning her actress just returned to that show) and became a main character there, ultimately becoming the Black Canary towards the end of Arrow Season 7.
Legends of Tomorrow: This is practically the entire premise of this series: Take a bunch of popular side heroes and villains from the various Arrowverse shows, put them in a team and give them their own show. That being said...it wasn't really until the second season that the show really took off and embraced its own sense of Crazy Awesome.
Season 3 does a little bit of both. It brings Wally West/ Kid Flash from The Flash as the newest member of the team, and it also brings back Damien Darhk, and lastly one of The Flash's most popular villains: Grodd!
The biggest addition to Season 4 is doubtless John Constantine joining the team, following a few previous guest appearances on Arrow and Legends season 3. Not only does it bring back John, but all of his baggage as well from the many unresolved plot threads from his cancelled series, enough for many to consider the fourth season being a Spiritual Successor or even a Sequel Series to Constantine.
Christine of Baskets. Originally focusing on failed French clown Chip, Louie Anderson's performance as Christine stole the show from the word go. Christine after season 1 is basically the main character, with Chip in a more supporting role.
Dylan Mckay doesn't even appear in the pilot and ends up being one of the most popular and iconic characters of the series, becoming the Trope Codifier for Troubled, but Cute and being involved in a love triangle with two of the female protagonists.
Kelly Taylor, to a lesser extent. She's probably more iconic than original female lead Brenda, she ends up stealing her Love Interest (fellow Breakout Character Dylan), and officially becomes the female lead after Brenda leaves the show. After Brandon leaves the show too, she's billed first in the opening.
The sequel series has Naomi Clark replacing Annie Wilson as the central character of the show starting Season 2.
The Big Bang Theory: Both Bernadette and Amy started out as sporadic recurring characters introduced in the third season and given their own little plot lines in the fourth. While they may not always show up unlike the core 5, they are credited as stars in the episodes they appear, and have appeared much more frequently than before. Starting from Season 5, they can be expected to appear in any given episode and get just as much focus as any of the other characters. Now they are integral part of the main cast with equal screentime.
Avon from Blake's 7. He was just initially supposed to be The Lancer to the titular Blake but Terry Nation (not to mention the audience) loved the spins Paul Darrow would put on his lines so much that by the second season Avon was a full-fledged Deuteragonist and when Gareth Thomas (Blake) left the series, it continued on for two more seasons with Avon as the protagonist.
Camille Saroyan in Bones was supposed to die in "The Man In The Cell" but she'd become so popular they kept her.
Saul Goodman of Breaking Bad was introduced as a comic-relief (though still highly competent) Amoral Attorney for Walter White, partly due to the fact that the creators knew that the show and characters were only going to get darker and needed some levity. Saul became an extremely popular character thanks to Bob Odenkirk's solid performance and charismatic, snarky attitude. So much so that even before the conclusion of Breaking Bad, he got greenlit for his own Spin-Off series, Better Call Saul.
Spike started out as an Ensemble Dark Horse, than his role got larger as the show went on and he became one of the major characters, which continued when he appeared in Angel. He even got his own miniseries in the After the Fall/Season 9 comics.
The entire ex-Sunnydale cast of Angel. They got their own show.
Arguably, Wesley is the most notable example. In his earliest appearances on Buffy, he was equally disliked by both the other characters and the fandom. Once he went over to Angel, he started to prove his worth and, after getting some excellent Character Development and taking multiple levels in badass, he became a much more popular character.
Jonathan. He was always shown as a generic Sunnydale student and later Ensemble Dark Horse in the earlier seasons. A later season has him return and turn out be a villain. But not really evil. In any case, more important than originally thought.
Anya. Originally a literal Monster of the Week, she was such an interesting character that she got a second episode outing, then was repurposed as Xander's rebound love interest. Joss Whedon did try to write her out on more than one occasion, but kept bringing her back. He waited until The Finale to kill her off.
Jill Monroe in Charlie's Angels. Despite the fact that she only starred in the first season, she is considered by many fans to be the most popular (for reasons not unconnected with her being played by Farrah Fawcett-Majors).
Phoebe in Charmed. She was really just there in the first two seasons as a background character but fans loved her and in season 3 she was given the main plot arc while Prue was shunted aside and eventually killed off when the actress left the series.
Leo Wyatt and Cole Turner also became breakout characters. However, Cole was eventually dropped in the middle of the fifth season.
Frasier Crane was originally supposed to be in Cheers for only a few episodes, but because of his popularity, he became a main character. When the series ended, he starred in a Spin-Off show.
Lilith Sternin also counts as originally she was only supposed to appear in one episode.
Lt. Columbo began life as a throwaway character from an episode of a short-lived anthology series called The Chevy Mystery Show. Here he was played by Bert Freed. That particular episode, "Enough Rope", was later remade as a Made-for-TV Movie titled Prescription: Murder, which marked Peter Falk's debut as the character. Even then, the murderer, played by Gene Barry, was envisioned as a Villain Protagonist with Columbo intended as a supporting character. Instead, Gene Barry's character became simply the first of many murderers to be outwitted by Lt. Columbo.
Community: The Dean and Chang started off as supporting characters, but as their popularity grew (and main cast members dropped out), they were promoted to the main cast.
J.R. Ewing was only supposed to be a supporting cast member, with the thrust of the storyline revolving around the feuding families. J.R. quickly stole the focus and the producers admitted he had become the breakout character of his series.
Cliff Barnes, Donna Culver, and Clayton Farlow to a much lesser extent as all three were only guest stars before they became part of the cast. Cliff Barnes ultimately appeared in more episodes than anyone else not called J.R. Ewing.
Barnabas Collins from Dark Shadows, who was originally slated to be staked within a few weeks of his first appearance.
Desperate Housewives: Orson Hodge went from being a one-off character, to a short-term villain, to a main character for 4 seasons.
Believe it not, the Doctor himself qualifies. True, he's the title character and his actor got top billing, but the producers had always intended for the show's protagonist to be Ian Chesterton, the dashing science teacher and man of action who served as one of the Doctor's first companions. The Doctor was envisioned as part mentor figure and guide and part instigator whose curiosity and alien interests caused trouble the others would have to resolve. Thanks to William Hartnell's authoritative performance, however, the Doctor quickly took over the show, and stories began to revolve around him starting with the second season.
It helped of course that all of the original cast members except for the Doctor left in the second season, leaving the Doctor as more of a familiar audience identification figure compared to all of the newer cast that came in.
Jago and Litefoot from "The Talons of Weng-Chiang" were so popular a TV spin-off was considered. This didn't happen but they got their own audio series, Jago & Litefoot.
Jack Harkness was so popular that a spinoff series followed not too long after.
Rory Williams is so popular, many viewers believed (with very good reason) that he would be the title character in "A Good Man Goes to War". And with the Doctor telling Kovarian that good men don't need rules and it's not a good idea to find out why he has so many, it basically was.
Sarah Jane Smith, who also later gained a spinoff.
K-9 not only became a regular in Sarah Jane's series, but also got his own spinoff series from 2009-2010. A previous spinoff proposal, The Adventures of K-9, had fallen through in 1997.
Jamie McCrimmon was only supposed to be a one-off character in "The Highlanders", but ended up traveling with the Second Doctor for all but his first serial, making him the longest serving companion in terms of consecutive episodes.
The Daleks were only intended to appear in the second story and nearly didn't appear at all, but changed the direction of the show from educational to Aliens and Monsters and became the most popular characters. There were even plans for them to have a spin-off.
The Weeping Angels dethroned the Daleks as the fan-voted "scariest monster" after their debut in "Blink", and became one of the most popular of the Doctor's rogues' gallery in the revived series. They went on to be the antagonists in two Eleventh Doctor stories ("Flesh and Stone"/"The Time of Angels" in Series 5 and "The Angels Take Manhattan" in Series 7), despite not having a major role in his particular Story Arc ( with the exception of the Cracks in Time). On the other hand, they haven't had more than cameo appearances since the Manhattan adventure to the point that the Twelfth Doctor went his entire tenure without confronting them. This could, of course, be due to the Hype Backlash fans experienced after three different Weeping Angel stories within a relatively short series of intervals; it's not uncommon to hear fans complain that the Weeping Angels lost more and more their scariness each time they reappeared.
Although companion Clara Oswald was extremely well-received and popular (becoming one of only a handful of companions to reach the same "beloved" status as Sarah Jane Smith), the fact she was conceived from the get-go as the most powerful companion of all time (befitting her status as the 50th anniversary companion) and Steven Moffat kept upping the ante with her to the point where she left the series as the Doctor's true Distaff Counterpart, technically ending her time on the series as a new incarnation of herself much like how the Doctor regenerates, resulted in negative response from some fans who saw her as The Scrappy instead.
Several members of the cast introduced later in the series also became breakout characters.
One could argue that several others from the first season rose in prominence, as well. Carol Hathaway is a prime example, as she was originally scripted to die from her suicide attempt in the pilot. However the test audience (and executive producer Steven Spielberg) enjoyed her so much that she was given a Promotion to Opening Titles in the second episode, became part of the show's supercouple (with Doug Ross, as it turns out), and won Julianna Margulies an Emmy in 1995 (the only one from the entire main cast to earn such an honor). She also earned more nominations than any of the other cast members (at six total, starting out as a supporting female before being upgraded to Lead Actress).
John Carter also received greater exposure as time went on, starting out as fourth-billed in the pilot's opening credits before becoming the central protagonist in the ninth season. However, this may have more to do with the fact that Noah Wyle was simply the only member of the original cast to remain up through then. Even so, Wyle received more Emmy nominations for his work than any cast member besides Margulies, and it's a testament to his character that the closing arc of the series was centered on his return to Chicago, especially as this was 4 years after Wyle departed from the show. "Set the tone", indeed.
Steve Urkel on Family Matters. He was originally supposed to be a one-shot character in the episode "Laura's First Date". By the end of the first season, Urkel was appearing in every episode, and when the first episode of Season 2 aired in September 1990, Jaleel White was given billing alongside the rest of the cast. The degree to which Urkel, the Extraverted Nerd, took over the show is hard to exaggerate. It's not uncommon to hear people refer to it as The Urkel Show. In addition, the fact that Family Matters started out as a spin-off of Perfect Strangers meant to spotlight Carl and Harriette Winslow makes this feel pretty absurd.
Family Ties was supposed to focus on the parents. The kids, especially Alex, quickly stole the spotlight.
Chiana was intended to die at the end of her first episode. Instead, she became a main character for the rest of the show's run.
Scorpius was originally intended as a one-shot villain for the first season finale, but proved such a hit with fans that he was promoted to Big Bad for the next two seasons and eventually became one of the protagonists in Season 4.
To a lesser extent Bulldog, he started off as a guest star, but fans liked him enough that he became a part of the main cast. However, the producers eventually dropped him when he started appearing in less episodes. He continued to appear occasionally.
King Richard of Galavant. A lovable, hapless doofus, the fans ate him up. When the show got a largely unexpected second season, Richard basically became the main character. With more narrative focus than even the title character. Down to being The Chosen One.
Lyanna Mormont quickly becomes one of the most popular characters on the show after only appearing in three episodes, and in one of those, she doesn't even have any lines, only a withering Death Glare of utter contempt for Ramsay.
Kurt became a fan and critic favorite even though he started off a supporting character.
Sue Sylvester, who wasn't even in the first script drafts until some fairly subtle Executive Meddling and even then wasn't intended as a major character. Importantly, Sue does appear in the very first scene of the first episode, but only as a one-joke character.
Santana is a clear example of a breakout character since she was supposed to be just Quinn's sidekick in the beginning and was mostly there to fill the requirement of 12 glee club members. Fast forward to season 3, when she has her own Coming-Out Story and then to season 4 and 5 when she's a main character and living in New York with Kurt and Rachel.
Blaine. Darren Criss was only originally signed on for a few episodes of season 2 to act as a mentor and potential love interest to Kurt. He became such a fan favorite that he was added to the main cast for season 3, and by season 5 he was a main character.
Zack Morris started off as a supporting character for Good Morning, Ms. Bliss with the title character, a teacher, being the actual main character. The series got retooled into being the super popular Kid Com, Saved by the Bell, with Zack being promoted to the starring role, and the rest was history.
J.J. Evans from Good Times. This was made even worse when two of the main characters were either killed off or temporarily gone, making J.J. even more of the focus of the show. The original star, Esther Rolle, actually left the show over it and only came back on the condition that they clean up the J.J. character.
Chuck Bass. The show originally gave him a similar role in the book as a date raping villain while also having him as Nate's sidekick. However his astounding popularity has led to him taking center stage to the point where Nate (who is the main male character of the books) seems to have mostly been shoved into the background while Chuck Bass is developed into a more likable character through his relationship with Blair and redemption plotline.
Blair is an even more prevalent example. Early on Serena was the main character but Blair quickly passed her for that title. It helped that Blair and Chuck's relationship became the show's Super Couple.
Sylar's popularity conflicted with the fact that season 1 was about killing him, so bringing him back sort of made everything from that season seem pointless and made his presence increasingly unneeded...
In the original publicity for the series, Niki is much more prominent than Claire (see the show's page image). Claire turned out to be one of the show's most popular characters; Niki... not so much.
Claire's stepdad Noah, a.k.a. HRG, was originally intended to be a supporting character, but he became the show's most popular non-superpowered human (and appeared in more episodes than anyone else except for the small blonde indestructible one).
And then with the reboot, not even her. Other characters from the original series also appear, but Noah is easily the most prominent member of the original cast in Reborn.
Archie Kennedy of Horatio Hornblower. He was written as an Ascended Extra and was meant only to appear in the first episode "The Even Chance". However, Jamie Bamber and his portrayal of Archie were so well liked by the cast and crew that his role was expanded to a Composite Character and he was written into "The Duchess and the Devil" and "The Frogs and the Lobsters", serving as awesome Foil to Horatio's Hero. However, Executive Meddling of The Forester estate would only let other parts of the Mini Series film if Archie Kennedy was to be written out. Not completely unreasonable, since their friendship was changing Horatio's characterization too drastically from the source. Archie was therefore given the opportunity to perform a Heroic Sacrifice to save Horatio in "Mutiny" and "Retribution", and thus immortalized himself in the process. In fans' (and perhaps even the authors') dreams, Hornblower would form the complete hero with Lt. Bush (Hornblower's canonical best friend from the book who appeared in "Mutiny" and "Retribution", and also in the final instalments "Loyalty" and "Duty") as the mind and Archie as the soul of the trio, à la Kirk's complete hero, Spock's mind and McCoy's soul. Even the star of the show Ioan Gruffudd expressed sincere shock that he would have never ever believed it possible to write Archie's character out of the adaptation.
Barney Stinson on How I Met Your Mother. He's often seen as the main character, even though it's supposed to be Ted. This actually ends up working in the sense that Barney's prominence does interfere with Future Ted's story about meeting his future children's mother... but it's completely realistic that Ted's life isn't conveniently tailored to somehow be completely thematically compliant with the story of the mother, and Ted can't cut out important parts of his life just because he wasn't the most important figure in those events. The landmark 150th episode replaced the opening with a gag opening called "How I Met Barney" to parody this trope, where Barney is in almost all pictures while Robin and Ted look annoyed at him.
Orenthal Gibson (Gibby) on iCarly. To the extent that he starred in one of two pilots for a potential spinoff (the one that didn't become a series, mind).
The first "Celebrity Jeopardy!" sketch on Saturday Night Live was written to showcase Norm Macdonald's Burt Reynolds impression. Darrell Hammond as Sean Connery was just to round out the players. But Connery turned out to be far more memorable because, unlike the other celebrities including Burt Reynolds, Connery was childishly antagonistic towards Alex Trebek in addition to being stupid. Connery ended up getting the funniest bit in the sketches (reading categories incorrectly) and Reynolds is barely remembered.
Snooki on Jersey Shore, which is an unusual example considering Jersey Shore is a reality show. The same thing happend with Zlatko in the first season of the German Big Brother.
When the show began, it was quite clear that despite the largish cast, Olivia Benson and Elliot Stabler were the central characters. Then fans started noticing Fin Tutuola. While the original pair stayed the main focus, Ice-T's Fin had several episodes and even a significant Story Arc and Arch-Enemy, sometimes to the point that the main pair barely made an appearance.
In season 14, Raúl Esparza came along for what was supposed to be a series of four guest-star episodes. His character, Rafael Barba, became so popular that it became a recurring role and then he was finally promoted to regular for season 15. He wound up staying until midway through season 19, making him the second-longest-running A.D.A. in the show's history (behind only Diane Neal as Casey Novak) and one of its three most beloved (alongside Neal and also Stephanie March as Alexandra Cabot).
Ben. He was originally meant to be a minor Other who would appear in only three episodes. Extremely positive reception made this extend to eight; then it went to being a series regular (the original leader of the Others was rewritten to be Ben) and one of the show's most important characters.
The same applies to Desmond: originally meant to only be in the first three episodes, his popularity made them bring back Desmond, focus the two hour finale on him, and make him a series regular the next year.
Klinger in M*A*S*H started as a one-note bit part in a handful of Season 1 episodes; by the end of the series, he'd been promoted to the main cast, replacing Radar as Company Clerk.
Melrose Place: Both Marcia Cross (Kimberly) and Heather Locklear (Amanda), with Kimberly stealing the crown in particular with her antics on the show...
Because of the episodic nature of Merlin, guest stars are seldom seen again after the episode in which they appear, however Queen Annis, Princess Mithian and the Dochraid (all introduced in series 4) were popular enough to ensure a reappearance in series 5.
Belle and Captain Hook from Once Upon a Time, both of whom started off in secondary roles but were elevated to the central cast due to their popularity.
Maleficient was also a One-Scene Wonder in the first season and became a main antagonist in the second half of the fourth season.
Also in the fourth season, Cruella De Vil became so popular that she was brought back as a heavily recurring character in the second half of the fifth season.
Brooke Davis on One Tree Hill. She goes from being comic relief character to stir the pot (who doesn't even appear in the pilot) to being billed third in the credits by Season Seven. She's also the character that most fans remembers first from the series above any of the Scotts.
Helena started off as an antagonist to her clone sisters, but her nuanced and complexity as well as her Adorkable knack for Black Comedy endeared her among fans. It could even be argued that she's the co-Series Mascot with main protagonist, Sarah Manning.
Patito Feo is an Argentine teen comedy freely based on The Ugly Duckling tale. "Patito" is Patricia Castro, an ugly girl, constantly insulted by the Alpha Bitch Antonella. But the aesop broke, and Antonella became far more popular than Patito, and the second season began fully focused on her.
Theodore "T-Bag" Bagwell, originally intended to appear in two episodes only. The character was so popular in those two episodes that he was promoted to main character and appeared in all four seasons, and even got to appear in a crossover episode of Breakout Kings after Prison Break ended.
Sara. To the extend that, when it was rumored that she may not be around for season 3, fans signed a petition to ask for her to continue being a part of the series.
Mahone had become so popular by the end of season 2 that the writers decided not to kill him off like they originally intended to, and keep him alive and locked in Sona.
From Red Dwarf, one Arnold Judas Rimmer. Interestingly this was completely unintentional on the part of the writers, who had always intended for the protagonist to be Dave Lister, the lazy yet lovable slob. But thanks to Chris Barrie's enthusiastic performance, Rimmer quickly took over the show, and more episodes began to revolve around him.
Guy of Gisborne from the BBC's Robin Hood was never a minor character (and the actor had top billing, behind only Robin himself and Marian until she was killed off), but halfway through the first season the writers develop a serious man-crush on him and his relationship with Marian. By season three, the show may as well be called "Guy of Gisborne."
Scrubs: The Janitor became a Breakout Character. He was originally only supposed to appear in the pilot episode, but Neil Flynn impressed Bill Lawrence so much that he was kept as a supporting character. It was then intended for him to be a background character to torment J.D. (if the show ended before the first half of the second season, he was going to be a figment of J.D.'s imagination), but the creators quickly (and thankfully) realized how awesome he was and included him in the main cast.
Kramer from Seinfeld: Seinfeld was originally supposed to have two main characters, Jerry and George. Kramer was going to be the wacky next door neighbor that pops in every now and then. After the first episode, the network wanted a main female character, and Elaine was brought in. So the show had three main characters, and was like that for the earlier episodes. As time went on, Kramer became more and more popular, and as a result appeared in the show more often as it went on. Eventually he became one of the main four, due entirely to fans wanting to see him.
Elmo from Sesame Street was a minor (originally unnamed) background Muppet in the show's early years before becoming one of the main focuses of the show and the Creator's Pet. Cookie Monster was originally a mascot for a defunct snack brand, and Grover was a minor villain in an Ed Sullivan Christmas sketch.
Benson from Soap the resident sarcastic character, who (when he left) received his own series, Benson.
Gannicus in the Spartacus series was written and included in the show prematurely as a result of Real Life Writes the Plotnote Lead star Andy Whitfield was diagnosed with cancer, thus delaying the production . Nonetheless, his cocky but good-intentioned "rock star" personality, coupled with his daredevil fighting style won him fans. By the final season, he has become the Tritagonist.
Stargate Atlantis's Dr. Carson Beckett was originally just billed as a supporting character but ended up in almost every episode of the first season, leading to the producers upgrading him to lead status for the second and third. They then tried to kill him off but fan attachment was strong enough that, in Daniel Jackson-esque fashion, he was brought back as a recurring character.
Interestingly, according to internet reviewer SF Debris, Neelix was originally created with this role in mind. In the series introduction in TV Guide, the producers state that this was indeed their goal. It didn't work out so well.
Star Trek: The Next Generation had Worf. Watching the first season it's clear he doesn't really have a whole lot to do other than being a Klingon hanging around the bridge and glowering at people to indicate the series was in The Future! compared to Star Trek: The Original Series. By later in the run entire main story arcs were devoted to the character, changes in the setting of the Star Trek universe (such as the Klingon Civil War) revolved around him, and that carried over to Deep Space Nine, which made him one of only two characters to be a regular in more than one Star Trek series.
John de Lancie's Q was the other one: the character is the only one to have appeared in all three of the contemporaneous Star Trek series in a major role in the episode (as opposed to a quick cameo).
In general, Worf, Data and Geordi essentially became a Power Trio of Breakout Characters in Next Generation, as their rising popularity led to them getting the highest number of Days in the Limelight between the three of them (largely eclipsing characters like Crusher, Riker and Yar, who got the spotlight much more frequently in early seasons). As secondary crewmen, the three characters' personalities contrasted each other well enough that each of them got to shine in their own ways: Worf, as the emotionally conflicted alien warrior from a foreign culture; Geordi, as the brilliant-but-awkward human engineer dealing with a physical disability; Data, as the stoic-but-optimistic android officer yearning to become human.
Introduced in "Hollow Pursuits", Lt. Reg Barclay was written as a one-shot character, but was so well-received that the writers brought him back throughout TNG and all the way to Star Trek: Voyager.
In Season 4, Castiel was only supposed to appear in a few episodes and then another character was to take his place as Dean's angel guide. However, general consensus decided he'd become the Ensemble Dark Horse. Even though he'd only gotten less than 40 minutes of screen-time at that point, the episode "On the Head of a Pin" (4.16) featured him heavily and 4.20 ("Rapture") centered on his vessel, Jimmy. In Season 5, Misha Collins (Castiel's actor) was promoted to regular status.
Bobby Singer was originally slated to be in only one episode, but circumstances led to him being given more appearances. Luckily the fans loved him. Fastforward to season seven and Bobby is one of the most frequently recurring characters on the show and Sam and Dean both admit to seeing him as the father John never was. Even after Bobby was killed off the writers have worked in ways to still get him to show up at least once per season, giving him the honor of being the only other character aside from Sam and Dean to appear in every season of the series.
Crowley was originally introduced as a minor villain who joined forces with Sam and Dean during the fifth season back when it was scheduled to be the final. He only appeared three times (once in the middle of the season and twice near its end) but quickly won over the audiences due to how cool, calm, manipulative, and cunning he was and the fact that he was played by Mark Sheppard didn't hurt either. When the series wound up continuing beyond the fifth, he was brought back as a recurring nemesis for the brothers. His popularity increased so much that as seasons went on his role was greatly expanded on even beyond that, and while several recurring characters were either written out or killed off he notably remained, and by seasons eight and nine he became the fourth most important character on the show after the brothers and Castiel, becoming an out and out Villain Protagonist opposing them. When the show reached its 10th season Mark Sheppard was promoted to a series regular, permanently cementing his breakout status.
"Reverend" Jim Ignatowsky on Taxi, played by Christopher Lloyd. The character first appeared in a one-shot role in the first season, being hired to officiate Latka's green card "wedding," but he was so well-received that the producers brought him back early in Season Two and made him a regular just a few weeks in.
Teen Wolf: Stiles was originally just Scott'sMuggle Best Friend and Plucky Comic Relief compared to the more prominent, dramatic characters like Allison, Derek and even Jackson, who had their own arcs and powers. However thanks to the audience's enthusiastic response and the (entirely inexperienced) Dylan O'Brien's acting talent, Stiles virtually ended up co-lead alongside Scott, their friendship became the cornerstone of the show and his background with his deceased mother and widowed father has been expanded. Season 3b even had him getting a whole arc dedicated to him being possessed by a thousand year old kitsune spirit and becoming the season's Big Bad.
For reference check out the difference between the Season 1 DVD art where Stiles isn't even on the front cover: ◊, to Season 2 where he's closest to Scott: ◊ and then Season 3b where he's front and centre: ◊
Similarly to O'Brien, but to a lesser extent, is Holland Roden, who plays the part of Lydia. She was mainly a tertiary protagonist in the first season, before getting her own subplot in the second and third (wherein she's revealed to be a banshee). By the fourth season until the series finale, she's arguably more important to the plot than the supposed main character, Scott, with many more of the subplots and the overall story arcs focusing on her and her newfound abilities. This focus, in turn, led her to become an Action Girl, which means she got even more screentime.
Kelso on That '70s Show. He began as one of the lower-tier main characters and later surpassed Topher Grace as the star. Seasons 6 and 7 even focused more on his character development than Eric's as he dealt with the trials of fatherhood.
Christopher Ellison as Detective Inspector Frank Burnside in The Bill. He was originally conceived only as a one-off character in the first episode, mainly as an outside antagonist for the main regular characters, but his performance so impressed the show runners that he was invited back to appear in two episodes of Series Two the following year. When original series regular John Salthouse (DI Galloway) left the show following it's third season, Ellison was invited to take his place, and the rest is history. Burnside became one of the characters most associated with the show for his Kubrick Stare and Deadpan Snarker qualities.
Sophia Petrilo of The Golden Girls was original intended to be an occasional Drop-In Character, but test audiences loved her so much that she was made an equal to the other three.
The Man from U.N.C.L.E.: An example of a successful Retool which made a minor character into a Breakout Character: The original premise had Napoleon Solo working alone, with Illya Kuryakin intended to be only a minor recurring character. Kuryakin struck such a chord with the show's fans, however, that he was raised to one of the leads.
Many of the character of The Muppets are this. Some examples include Kermit, who started off as a supporting character on the show Sam and Friends before eventually becoming The Muppet's unofficial leader and Gonzo, whose role has expanded over time, especially in the movies, where he is one of Those Two Guys or the Lemony Narrator, but later had an imaginary film regarding his origins/species, in which he was the protagonist. He also had an expanded role in Muppet Babies as a Dogged Nice Guy toward Miss Piggy.
Miss Piggy herself started out as a bit player in a Herb Alpert and the Tijuana Brass TV Special, before getting larger and larger roles on The Muppet Show and being just behind Kermit.
Uncle Deadly was introduced in one episode of The Muppet Show as a ghoul who haunted the Muppet Theater. After that, however, he floundered in obscurity for years, though he was planned to have a spinoff that never got off the ground. He eventually made his big comeback in the 2011 film, and has a recurring role in the new TV series as the manager of Miss Piggy's wardrobe. He even has his own Character Blog on Twitter.
Jaime Summers, The Bionic Woman, was meant to be a one time character in a two-part episode of The Six Million Dollar Man, to the point where she died at the end. However, the character's popularity caused the writers to retcon her death and give her a spinoff.
The Thick of It. For the show's first two seasons, Hugh Abbott was clearly the main character and focus. After his departure at the end of Season 2, several previously secondary characters saw their roles significantly increased to fill the gap. Malcolm Tucker became more and more prominent as the show went on.
President Bartlet was only supposed to be in four episodes a season, with the show focusing on the White House Senior Staff. But when Martin Sheen's performance wowed the production team, the show was retooled to make Bartlet a main character.
Donna Moss was intended to be a minor character, on level with the other assistants on the show. Bradley Whitford (who plays Josh, her boss) pointed out their fantastic onscreen chemistry to the producers. This led to Donna being upgraded to a main character with her own storylines, and replacing the character Mandy as Josh's main love interest.
Chloe is a unique example, as when she was originally introduced the fans initially hated her, finding her sarcastic streak that she used to put down her co-workers annoying. One season later and her personality was reworked a bit, making her much more sympathetic while also keeping her deadpan wit from before, but this time around having it focus on characters that deserved it. This new portrayal of her was much more well received, so much that she won enough popularity to return midway through the fourth season after initially being written out of it early on. Then the moment of truth came where she wound up getting attacked and fighting the attacker off with an M16, which quickly cemented her as a fan favorite. She quickly wound up becoming the second most important character on the show after Jack, so much that she was the first non-Jack character confirmed to return for the sequel mini-series.
While Tony was on the show from the very beginning he was originally just a recurring character. As the first season progressed and Tony went through some Character Development that made him go from being Jack's rival to friend, he quickly ended up becoming a hit with the audiences, so by the time the second season rolled he was quickly promoted to the main cast and ended up becoming the second "main" character after Jack. Attempts at trying to write him out of the series always ended up failing, to the point that when he was controversially killed off in the fifth season, the fan outcry was so high that his death was later undone. Even after he finally made a permanent exit at the end of the show's seventh season, the demand for his return led him to make a return appearance in a short feature on the home release of said sequel mini-series, which eventually lead to the later confirmation that Tony would be returning in the sequel/soft reboot show Twenty Four Legacy to help link it to the original series that would otherwise feature a completely brand new cast.
Claudia was something of a Tag Along Kid and Gadgeteer Genius who could have easily became The Scrappy of the show, but the writing managed to make her endearing enough for her to be Promoted to Opening Credits. She then became the destined next in line to become Caretaker, a full field agent, and become a core point of several of the show's biggest storyarcs. Also came with something of a downside as the last season felt the need to make her backstory more complicated, which had a negative effect on her character.
HG Wells to a lesser extent; originally intended to be the Big Bad of the second season, she was brought back in the third as The Atoner due to the fandom's love for her character, leading to her then becoming something of a freelance agent of sorts. She did get Put On The Bus in Season 4, but the finale brought her back one last time in flashbacks.
Rico was so well-received from his few appearances in Season 1 of Hannah Montana that he became a main character from Season 2 onwards.
Orange Is the New Black inmate Crazy Eyes was hugely popular with audiences in the first season, and became one of the characters who received a lot more emphasis and development when the show returned.
Tiffany (Nikki Cox), the sexy daughter on Unhappily Ever After, became so popular that starting in season 3, almost all the episodes were about her.
Chito Manaloto (Jhake Vargas) in Filipino sitcom Pepito Manaloto. Much like Ricky Nelson became a bigger star than his titular parents in The Ozzie and Harriet Show, the same appears to be happening to Vargas, the good-looking teenage son in the series, as most of the main storylines center on him and not his dad Pepito (Michael V).
Charlie Brooker created Philomena Cunk to be a thick-as-a-brick 'posh TV historian' talking head for his Newswipe and TV criticism shows. Partly because of the actress Diane Morgan's amazing performance, she ended up being popular enough to star in her own Moments of Wonder segments on his shows, and then appear in hour-long TV specials Cunk on Shakespeare and Cunk on Christmas.
The Great British Bake Off: Season 6 Winner Nadiya. People quickly fell in love with her exaggerated faces and sweet personality, and since winning has become a media personality, TV host (Including two cookery shows and Junior Bake Off) and author, as well as maintaining a baking career, even baking a birthday cake forThe Queen.