Chloe O'Brien. Especially interesting as she was hated by the fandom until she started becoming awesome halfway through season three.
Apart from Jack Bauer (who's the main character anyway), the only other character to appear in the first seven seasons of 24 is Aaron Pierce, whose actor (Glenn Morshower) was listed as a "guest star" rather than a member of the main cast in the 49 episodes he appeared in. He is also probably one of the few characters who can rival Jack in terms of Badass. (It's also worth noting that despite not appearing in the final season, he was not killed off.)
Tony Almeida, who was pretty much Jack's antagonist in season one. Then he is the one to receive information that Jack's wife Teri is suffering from amnesia and is in trouble, and they milk the situation making the audience worry that Tony is the infiltrator at CTU and will let Teri die. He then shows up at the Bauer residence and shoots a man about to kill Teri, earning both the audience's love and Jack's trust. Since then Tony has been one of the most beloved characters on the show.
Renee Walker has become one. Originally introduced in season seven as a foil and Morality Pet for Jack, she's gone on to become a bona fideBadassKnight in Sour Armor, and one of the few people on the show capable of out Jack Bauering Jack Bauer. It's little wonder her nickname in fan circles is "Jill Bauer".
Mandy, who only appeared in 7 episodes in the series (split between Seasons 1, 2, and 4), yet made an indelible impression as a Dark Action Girl who always knew how to get away scot-free.
Good old Bill Buchanan. A Reasonable Authority Figure who trusted Jack's judgment (even if he disagreed with his methods) and had the clout to help Jack out when the situation called for it. Easily the most competent CTU Director the show ever had.
Belcheck from Day 9 appeared in most of the season but didn't have a lot of screentime. What little he did though showed that he was as much of a badass as Jack was, and the fact that he's one of the rare characters late in the series to show 100% loyalty to Jack quickly earned him a lot of popularity.
Kenneth, who becomes more and more prominent in each season.
Jack Donaghy as he was originally supposed to be a minor character, but is one of the leads along with Tina Fey.
In Alphas Gary Bell, the high-functioning autistic member of the group, is by far the most popular character in the series. Amongst the many reasons he's so beloved is his cool power, his snarky attitude, and the fact that he almost always has the best and funniest lines on just about every episode.
Ron Moore and crew famously intended to leave Helo stranded on post-nuke Caprica after the miniseries, but were besieged by viewers wanting to know how he was going to survive: ironically, his rescue has led to several major plot points. The show also features a strong and relatively unchanging core cast, many minor members of which have become popular in fandom.
Cally. An extra in the loading bay, she got picked for a violent death after several scenes in the background. She was supposed to be raped then killed. Instead she bit off the attacker's ear and got promoted in status. Eventually she marries a main character.
Anders grew from a romantic foil for the Lee/Kara ship to a main character. He gets to be part of Galactica's final send-off and even appears on the Season 4.5 DVD boxset.
There's Racetrack, who was meant to be a one-episode bit part replacement for Boomer's co-pilot Crashdown who was kept on as a recurring character simply because the producers liked the actor who played her.
Seelix, who's something of a Manufactured Darkhorse by none other than Aaron Douglas, aka Chief Tyrol. Her character was literally credited as "Technician #2" in her debut episode, but Douglas began calling her Seelix during takes, which she was eventually credited as. As the show went on, she got a first name, and became a Viper pilot—All because Aaron Douglas is a Bad Ass actor.
The Big Bang Theory: This show was originally intended to revolve around a small, bemused nerd named Leonard Hofstadter. Then we met his egotistical, obsessive-compulsive, yet still strangely lovable roommate Sheldon Cooper, and the rest is history.
Bionic Woman: In the remake, Katee Sackoff's Sarah Corvus, "the first bionic woman", ran so many rings around the supposed lead that many people watched the (dreadful) show for her alone.
Blake's 7: Avon, the snarktastic, outwardly amoral, leather-wearing computer geek, becomes the lead in series 3 and 4. Also, Vila; probably intended to be the most minor of the original Seven, he became the only character to be in every single episode.
Chalky White, played by the same actor as Omar Little on The Wire. Michael K. Williams has such a small, but memorable part in the pilot that fans latched onto him. He's back as a regular for season two, as well as getting a few episodes in the spotlight. By season 4 he's practically the show's second lead.
Ascended Extra and hitman with a mysterious past Mike started out as a one time character before becoming a scene stealer and eventually becoming a central part of the ensemble towards the end of the show.
Bob Odenkirk's sleazy Plucky Comic Relief lawyer Saul Goodman often has the funniest lines in the show and though he doesn't appear in every episode he's always memorable and there has even been talk of giving him a Spin-Off, rare for a character on a basic cable drama.
Spike (pictured) went from single-season (originally intended to live five episodes) villain to main cast member eventually leading to full-fledged spotlight stealing. In the final episode, he sacrifices himself to save the world. Then he came back in Angel, which was still running, and started receiving at least as much screen-time as the rest of the cast.
Anya went from a single-episode villain to main cast member.
Reportedly, both Tara and Joyce (Buffy's mom) were supposed to die well before they did, but the writers kept putting it off because they were liked by the fans. Tara's death in particular is a cause of much Fan Disillusionment with the series.
Oz was originally slated to die in the second season, but managed to avoid death because of fan support.
Wesley Wyndam-Pryce first appeared as Faith's Watcher and was intended to be killed off shortly after. In the words of actor Alexis Denisof, Wesley was supposed to "come in, irritate Giles and Buffy for a couple shows, and then be gloriously terminated". However, the writers grew fond of him and kept him around. When Joss Whedon added Wesley to the main cast of Angel, he quickly became a fan favourite.
Faith was originally supposed to be there for five episodes but managed to be such a fan favourite that she lasted right until the end. She would have even gotten her own show if Eliza Dushku hadn't declined in order to work on Tru Calling.
Harmony was one of Cordelia's dumb, snobby friends in the first season, was turned into a vampire for season four, and eventually became a regular on Angel.
Amilyn is arguably most popular character of the film, due to his death scene.
Burn Notice: Madeline, Michael Westen's mother, in more ways than one. She started off as a thorn in Michael's side, and an annoyance to viewers; however, only a few episodes in, she started looking tougher than most of the bad guys. Since then, she's befriended every regular on the show, done her fair share of espionage, reconciled Jesse and Michael after their falling out, and been the only one other than Fiona whose threats Michael seems to actually fear. So of course, she's earned herself a following among watchers that's roughly proportionate to the respect she's earned from the characters.
Castle: Lanie Parish is becoming this. She can most definitely hold her own against Castle and Beckett, and seems to get some of the best lines in an episode that aren't uttered by the two leads.
Casualty: A blonde nurse, only ever referred to as "Cath" or "Kath", seems to be popular with the fans, yet never really gets any storylines (not even a minor one). She's not a One-Shot Character or a Ghost Extra by any means, but she rarely speaks. But, amazingly, she is never credited on-screen. She is blonde and appears mostly in scenes where Zoe, Ruth, Jay and/or Nick Jordan are present. Here's a photo of her, for those wanting to see◊.
Cole Turner was written out of the series midway through season 3 and yet was brought back and kept around for two more seasons because fans liked him.
D'Eartha (the seer) was only on for ten episodes and yet nearly everyone loved her. Mostly because she was always snarky, awesome and deliciously evil but partly because she had more chemistry with Cole AND the sisters than any of the proper pairings were supposed to have.
Kristen Kreuk (known best for her role as Lana in Smallville) makes a brief guest appearance as "Hannah" in Series 3 as a potential romantic interest for the main character, altering the dynamic of his will-they/won't-they relationship with Sarah Walker They do.. Her characterization ended up as being a much more likable, well-rounded, and interesting person than Sarah but Chuck still dumps Hannah. It wasn't that she was more well rounded than Sarah, who was a well rounded character in her own right. It's more that she was cute and funny and had some chemistry with Chuck. However seeing as the majority of the shows fans are Sarah/Chuck fans, and the series up until that point had built up the relationship between them, the switch would not have gone down well.
Ellie's boyfriend Devon was originally going to be a spy, but his character was too popular so they kept him as the lovable Captain Awesome. Though they kind of got to have their cake and eat it too, as he's still the first major civilian character to learn about Chuck being a spy, and gets to help him quite a few times.
Come Fly With Me: Precious Little, from this British Mockumentary, quickly became the most popular character due to her catchphrase and whacky attempts to get out of working (Example: sawing the pipes so the plumber would have to repair them.) To go shopping or some other activity.
Annie is the viewer crush and romantic darkhorse, quite literally, as she gets in the way of the Britta/Jeff and Jeff/Slater ships in the finale. She wins. For now at least.
Señor Chang for his outlandish outbursts and randomness
Dean Pelton for being incredibly over the top politically correct, whilst having a Dalmatian/furry fetish and Ho Yay with Jeff.
They often form a Darkhorse Ensemble, like in Modern Warfare where Chang was The Dragon to the Dean's Big Bad.
The Cosby Show: While many people see Pam Tucker as an example of The Scrappy, you can also see her as a welcome bit of fresh air in a show, that would last only two more seasons anyway. So she's more of a Base Breaker. Despite how the show always was set in New York, it wasn't until when Pam showed up in the 7th season, that we got a regular view of what it was like to live in a ghetto neighborhood. She had obviously had a rougher life than Cliff's and Claire's own kids, and she could be seen as much more down to Earth than her cousins.
Among the villains, Tobias Hankel is very popular. He was very well acted, screwed-up in a particularly fascinating way, and ultimately rather sympathetic. His popularity also probably spawns from the importance of the episodes he was in, being a crucial element to one of the main's Character Development.
Dark Angel: Alec was very popular with fans, particularly female ones, who often watched more for Alec character development than for the Familiars vs. Transgenics storyline. Alec's appearance on the show was determined by the popularity of his genetic twin, Ben. Despite being mentally unstable and homicidal, he gained a lot of popularity in the one episode he appeared in. Since he is killed at the end of said episode, the writers decided to bring the actor back as Ben's (equally charming, slightly less unstable) twin, Alec.
Dark Shadows: When Willie Loomis freed vampire Barnabas Collins from his coffin on the original show, all the writers had meant for him to do was chew on a few expendable extras, then get staked during sweeps week. Between dressing like a Badass Longcoat, radiating Cursed with Awesome, and being the only character of his era whom the censors permitted to neck on television, Barnabas raked in so much fan mail from kids and (especially) housewives that he saved the show from cancellation, as well as becoming the show's central and best-remembered figure. So who's your Daddy, Angel, Mick and Nick...?
Dawson's Creek: Pacey Whitter, who was merely supposed to be Dawson's wise-cracking best friend but proved so popular that more storylines were built for him, culminating in him getting the girl at the end of the series over Dawson.
The Daleks definitely qualify. In 1963, nothing even remotely resembling them had ever been seen before, and their stellar popularity caused them to be upgraded from one-time Monsters Of The Week to return appearances in season two and season three. They were Killed Off for Real in season four, but by that time, Joker Immunity had firmly set in. In fact, the series' developers originally wished to avoid stories about bug-eyed monsters or robots, intending to tell more grounded stories teaching children about science and history. They were reluctant to make "The Daleks" at all, yet it was the serial that made Doctor Who a success. They've grown into their hype, becoming gods of destruction capable of almost making universe-destroying plans work. Massive armies descending on the universe and destroying all in their path is common. And it is awesome.
Colonel Lethbridge-Stewart was originally intended as a one-shot character in The Web of Fear, and Jamie MacCrimmon was originally intended as a one-shot character in The Highlanders. Colonel Lethbridge-Stewart became Brigadier Lethbridge-Stewart, companion at one time or another to every single Doctor in the Classic Series, longest-running companion in the show's history, even showing up on The Sarah Jane Adventures. Jamie, meanwhile, went on to become the Second Doctor's longest-serving companion, featuring in all of his stories bar one (his first, The Power of the Daleks), and making return appearances in The Five Doctors and The Two Doctors.
Weeping Angels. Steven Moffat made them up on a whim, and they are now considered to be the scariest and best new series monster in the opinion of many (including Neil Gaiman). After seeing the episode "Blink", you'll actually hesitate to blink while looking at a statue...
Jeff has appeared in one episode, and been mentioned in another, yet somehow has quite a large fanbase.
Wilfred Mott. Initially intended as a one-shot newspaper vendor called Stan, rewritten as Donna's grandfather before becoming the Doctor's companion. Also the Tenth Doctor ultimately dies saving his life
Rory Williams is either one of these, or a blatant example of the writers deliberately manipulating a character to make them go in this direction: from someone who looked like he was going to be a dorky Mickey Smith clone, to a character with far more appeal (right before killing him), to a Bad Ass and Unbroken Vigil.
Canton Everett Delaware III is quickly shaping into one of these. After Day of the Moon the fandom is pretty much divided between those that want to see him return, and those that that want to see him return as a companion (Of course, he is played by Mark Sheppard.)
The episode "A Good Man Goes To War" brings us Jenny and Madame Vastra. What’s so awesome about them? Well, Madame Vastra is a female Silurian. Living in Victorian London. In a mansion. And she works for Scotland Yard, hunting down criminals. And eating them. She’s in a lesbian/inter-species relationship (later full-on marriage) with her kick-ass maid-servant Jenny, and they both wield katanas. They’re simply amazing, and we didn't even get to see how they met the Doctor, and it’s doubtful we ever will.
The same episode introduces Strax. Every moment with him is hilarious, and many fans (along with the cast) strongly believe that he, Vastra, and Jenny deserve their own spinoff show (especially once they became recurring characters throughout series 7). Moffat considered making one, but decided he didn't have the time.
The Special Weapons Dalek only appeared in one serial in the classic series, but is very popular due to being so damn cool (to the point they brought it back for the new series' seventh season.)
Sally Sparrow. She gained a lot of fans after her appearance in "Blink". More than once, it's been suggested that she be the next companion. Of course it doesn't hurt that Carey Mulligan, the actress playing Sally, is extremely attractive.
It helps that Mulligan has gained more popularity outside of Doctor Who as a rising Hollywood actress, with an Oscar nomination for her work in An Education and much acclaim for her work in films like Shame, Drive, and The Great Gatsby.
Brian Williams, Rory's dad, seems to be going down Wilf's route.
For "The Crimson Horror", despite being a parasite, Mr. Sweet actually won many viewers' hearts due to his cute name and face. In fact, when Ada kills him, some viewers were actually sad and wished the Doctor should have saved him and returned him to his timeline.
Jethro, from the episode Midnight, is loved by fangirls for his facial expressions and his dry, sarcastic attitude. Being played by the guy who would eventually play Merlin helps, too. More than a few fans are hoping for him as a companion for the Twelfth Doctor.
The Zygons have proved to be a popular creation even though they only made one televised appearance in the Classic Series, in 1975. David Tennant has said they are his favourite monsters. The Zygons were so popular that they were the main villains in the 50th Anniversary Special.
Journey Blue from Into the Dalek. Not ten minutes after the end of the episode's premiere, there were already fans calling for her to become a companion, not to mention shipping her with Clara.
In the pilot, "24 Hours", Carol Hathaway's suicide attempt was supposed to have succeeded. But Carol was so loved that in the second episode, "Day One", set a while later, we find out Carol survived. She stayed for six seasons on the show and is one of the most beloved female characters from it.
In the 5th season, Carol Hathaway has a short scene with Nurse Abby Lockhart (Maura Tierney), credited at the end of the episode. Tierney then becomes a main actor in the 6th season and slowly moves up the credit order (as other actors leave) to become top credited in the first half of season 16 (until she leaves) and the central character of the series for several seasons leading up to that.
Family Matters: Steve Urkel started out this way; the series moved from being about a middle-class black family with a Wacky Neighbor to the Mad Science adventures of Urkel and his attempts to win Laura's heart. Even becoming a Wacky Neighbor was due to his Darkhorse nature. He started out as a one-shot character, set up as a blind prom date for Laura by her father, the single most harmless, nonthreatening guy he could possibly find. The studio audience fell in love with the character. By the end of the episode, they were chanting "Urkel! Urkel! Urkel!" at his entrances. In the next episode, Urkel was retconned into being the Winslows' next-door neighbor, and Family Matters shot up in the ratings to become ABC's flagship show.
Family Ties: Alex Keaton. The Dom Com was designed as a star vehicle for Meredith Baxter-Birney, formerly one of the daughters on the drama series Family. However, Michael J. Fox's performance, as the over-the-top Republican foil to his former hippie parents, won over the audience such that most of the plots soon centered around him.
Chiana was originally planned as a one-episode character - she is injured in her first episode when a Pulse Pistol blast skims her arm, but the writers originally envisioned the shot killing her. Not long after, she became a cast regular.
Scorpius was just supposed to be a one-episode villain: however, impressed by the results, the producers decided to make him the Big Bad for the show- and actually made it work for two seasons without Villain Decay setting in!
Firefly: While River Tam appears in every episode, she usually plays a fairly small role, but is still considered the main character by many of the shows' fans. It is partially helped by the fact that she gets three of the fourteen episodes of the series focusing on her, plays a major role in two more, and is one of the two main characters in the Big Damn Movie. While River did appear in every episode, she often got far fewer lines then the other characters and in the case of "Our Mrs Reynolds" none at all. She is STILL considered the main female character, and rightly so, because her tragic story was one of the show's Myth Arc storylines, and her character and backstory would have been explored more hadn't the show been cancelled.
"I can kill you with my mind." Sufficient unto itself.
FlashForward: Demetri. A main character-specifically the lead's sidekick-the most common complaint about the show was that the roles of Demetri and Mark should have been reversed. His planned death was actually rewritten to keep him alive due to his popularity.
Flashpoint: Spike and Lou seemed to serve this role through the first two seasons as we really don't see anything of their personal lives and they are never the focus of an episode. This changed in the series 3 premiere when Lou sadly dies and Spike has to deal with the aftermath. Perhaps this gave the writers a chance to see that Sergio Di Zio can do angst just as well as he can do goofy and snarky, because it seemed like Spike got a whole lot more to work with, story-wise, after this.
Frasier: Dr. Frasier Crane, rivaled only by The Fonz. He was originally created for a one-season arc in Cheers as Diane's love interest, but due to his popularity was brought back for all subsequent seasons as a regular character. He would later go on to have his own spin-off that would last as long as Cheers (for a total of 11 seasons) and launch Kelsey Grammer's career with significant star power behind it. To clarify, that is 9 seasons in Cheers and 11 on Frasier. He was introduced in Cheers in 1984, and Frasier ended in 2004. Twenty years, for a single, originally one-season character. Come to that, there's also Frasier's brother Niles, whose well-meaning but uptight personality turned out to be winning and helped make him the The Woobie of the show.
Both Phoebe's love interests, Mike and David, are well liked, especially Mike.
Jack Geller is popular too. His wife not so much...
Janice is this. Although, being hilarious and annoying at the same time, she's also a Base Breaker.
Richard, for being one of the most decent and likable love interests. Like Janice though, he can fall into the Base Breaker territory, since some people see him as The Scrappy.
Kathy was this, at least until the writers broke her and Chandler up. The cast actually liked Paget Brewster enough that they wanted her to stay.
General Hospital: The ultimate Soap Opera example has to be Luke. He was originally intended for a short term role as the lowlife brother of bad girl Bobbie and even raped beloved heroine Laura. However, the actor not only had a lot of charm and talent but also amazing chemistry with the actress who played Laura. They became the Trope MakerSupercouple despite the controversy and even today Luke is arguably the show's most iconic character.
Blaine. His first musical number sold more copies on iTunes than any other song ever performed on Glee, and to this point he's had more solos than many of the main characters due to his popularity. He's played by Darren Criss, who had his own large fandom before he even joined Glee.
Dave Karofsky, along with the actor who plays him, Max Adler, is incredibly popular particularly among gay male fans. There is a fan campaign going around that wants Karofsky to sing on the show, as well.
Coach Shannon Bieste, the only character on the show whom the vast majority of the fanbase unabashedly loves. Even those who hate literally everything else about it love Shannon.
The Golden Girls: Sophia Petrillo, Dorothy's elderly mother whose post-stroke status left her with no filter between her brain and her mouth giving her a tell-it-like-it-is attitude and biting wit, was only supposed to be a semi-recurring character (in the pilot episode, she moves in after her nursing home burns down and the ladies also had a gay male cook). She was so popular from that initial appearance that the cook was written out of subsequent episodes and Sophia was given his snappy lines. Sophia was also part of the cast of characters in both of the show's spinoffs.
Gossip Girl: Chuck Bass could be the poster boy for this trope. In the books that the show is based off of, the character is a sociopathic, bisexual rapist who no one likes. On the show he was originally supposed to be a supporting character appearing only a few times during the first season, and started off his run on the show by attempting to rape two out of three female leads in the pilot episode. It only took a few episodes for him to be bumped up to main character, and he's now been transformed into the Tortured Anti-Hero and gained a rabid following among fans. The creator of the show admitted to redirecting the focus towards the relationship between Chuck and one of the female leads (the one he didn't try to rape), stating "we know the Chuck and Blair thing works for people." As 'Chair' (Chuck and Blair) he is now one half of the show's central relationship (the producers of the show have called Chuck and Blair the king and queen of Gossip Girl.)
For that matter, what about Hiro himself? He was added as an afterthought when the creator's wife pointed out that no character created so far actually liked having powers, and it's partly this very love of being superpowered that has made him easily one of the most (if not the most) popular characters.
The Bennet family's son Lyle and dog Mr. Muggles also qualify, and the latter has become more of an in-fandom meme.
Highlander: Methos. He was originally supposed to die at the end of the third season, but he stayed alive for the whole series and the two movies that came afterwards because he was a terriblypopularcharacter. So popular, in fact, that some fans regularly remind others that Duncan McLeod is supposed to be the hero of the series (people tend to forget that).
Holby City: This Spin-Off of Casualty has an Ascended Extra who is occasionally used in storylines and is more than a Living Prop or Ghost Extra. The show has a ginger-haired nurse called Marie-Claire, who speaks with an Irish accent, making an appearance. Her accent is genuine Northern Irish, and no Oireland stereotypes are used here.
Horatio Hornblower adaptations: There are several fan favourite minor characters, and some fans prefer them even over the main man Horatio.
Gushing for Archie Kennedy has no limits. He was originally intended only to appear in the first episode, but worked so well as a more extroverted foil to the withdrawn protagonist that he appeared in five out of the first six TV movies. The fact that he's The Woobie, and that he and Horatio are very cutetogether probably helps his popularity with the fanbase. His cause probably wasn't hurt by the fact the first four movies were based off the book Midshipman Hornblower, set before Hornblower's canon sidekick Lt Bush turns up. Indeed the fifth and sixth movies have both characters as very different styles of companion for Hornblower before Kennedy dies and Bush takes over for the rest of the series, as fits the books.
HisLordshipMajor Edrington appeared only in one of eight installments but became very memorable and fans' particular favourite. His cool is beyond awesome and his Gentleman Snarker character shines very brightly, even though the series is the World of Snark. He's very popular in fan fics.
The Duchess from "The Duchess and the Devil". She's an older lady, but vivacious, spirited, cheeky, clever, charming and immensely hot, capable of taking care of herself and able to fool the Spanish by hiding Horatio's super important dispatches. It doesn't happen a lot that a hero of Wooden Ships and Iron Men adventure gets paired with such an interesting and well-developed woman counterpart. Most fans absolutely adore her, though she has also been called "annoying cargo".
Captain Pellew is Horatio's mentor and his role was intended to be substantial from the beginning, but Robert Lindsay's portrayal raises the love for him above the intended level. Fans agree that he is the definitive Captain of the series. However, his lack of interest in Archie and some of his actions in "Retribution" are questioned and not welcomed by all fans.
Jason, by a lot. Despite being a teacher and one of the secondary characters, when he didn't come back after Season 1, many fans were and are still begging and waiting for his return, probably making him one of the most loved characters in the show. This might have something to do with his 'relationship' with Patricia.
How I Met Your Mother: Has Barney Stinson, who might very well be the reason for the show's success. His character even got two spin-off books, and is the feature of many commercials. To people not familiar with the show, Barney is often believed to be the main character.
Ranjit, Sandy Rivers, Marshall's dad and Ted's love interest Victoria also count
Wendy was a mostly semi-recurring Mr. Exposition character, who gained enough popularity to start being shipped with the main characters. Wendy was a victim of Chuck Cunningham Syndrome at the end of Season 2, roughly the same time Gibby started to be used more frequently.
Gibby started to become more involved in the jokes and storylines, and in Season 4 became a full part of the main cast.
In-Universe, Freddie is one. When they interact with their fans at Webicon, he is mobbed by girls. Carly and Sam don't get anywhere like that level of attention, and most of their attention is based around which of them should be dating Freddie.
Justified: Boyd Crowder. He was originally going to be a one off villain. Raylan shooting him was supposed to kill him. However, the showrunners fell in love with Boyd and the fatal shot was changed to "Near fatal" and Boyd is a show mainstay now.
Kamen Rider Den-O: The DenLiner Imagin - Momotaros, Urataros, Kintaros, and Ryutaros - and Deneb were just the Non Human Sidekicks to the show's main heroes, but by the end of the year, their popularity had overtaken the show itself. Mostly the Taros'. This prompted several musical releases, a spinoff anime starring them, and three additional movies for the series (one of which is part of a larger series.)
Kindred: The Embraced: Mark Frankel's Julian Luna became this, after poor writing and acting resulted in lead character Frank Kohanek (C. Thomas Howell) becoming The Scrappy. Season 2 was going to make Julian Luna the lead and write out Kohanek; but Frankel's death in a motorcycle accident resulted in the show being cancelled after the first season, as no other characters were strong or popular enough to carry the show.
Crown Prince Jack Benjamin. When your daddy's played by Ian McShane, and you're still stealing scenes, it's safe to say the fans are going to adore you. It helps that Sebastian Stan is fairly well known for his role on Gossip Girl and is really rather attractive.
In fandom, not only is Jack one of the most popular, but his stalker / secretboyfriend Joseph (who appeared in only two episodes) gets more attention than some of the main cast.
The Late Late Show: An almost literal example with Secretariat with Craig Ferguson. Initially just stock footage of some goofballs gadding about in a horse suit, the pantomime horse soon began to show up in the studio as a running gag and proved extremely popular with audiences, to the point where they've even had "Team Secretariat" T-shirts made and a whole sketch ("Secretariat in New York") was created around the character.
The League of Gentlemen: Papa Lazarou only appears four times over the entirety of the show's run (including The Movie), and yet he's probably the show's most discussed, quoted, and beloved character.
The show has Massachusetts State Police detective Patrick Bonanno. He made a few appearances in season 2, most importantly serving as the impetus for the events of the season 2 finale. The writers were really surprised at how happy viewers were to see him appear in "The Jailhouse Job," the season 3 premiere.
It could also be argued that Hardison's rival hacker "Chaos" is an ensemble darkhorse; he is the only one to re-appear from "The Two Live Crew Job" (as of s3), and the fans were glad to see him again. This is particularly ironic when considering that he is played by Wil Wheaton.
Life on Mars had Ray and Chris, who played small rolls until they were upgraded to regulars on the sequel series Ashes to Ashes and then promoted to main characters in Season 3.
In the finale infact, it is revealed that Ray and Chris also came from the real world, and presumably underwent a similar story to Sam and Alex preceding Life on Mars
Lois and Clark: Back in the 90s, this show had a problem. Like many Superman shows, the hero suffered from a lack of competent foes. His biggest enemy, Lex Luthor, was written out of the show after actor John Shea opted not to return for the second season. What to do? Along came Tempus, time-traveling bad guy and one-shot villain. Played by obscure soap actor Lane Davies, Tempus' endlessly-quotable jibes and penchant for Leaning on the Fourth Wall proved so popular that he ended up returning each subsequent year.
Both Ben and Desmond. Each was intended to have only a short arc, with the door left open to become a regular. Both proved popular and became important characters.
Rose and Bernard. They began as rather minor characters, with Rose being a blatant Magical Negro in season 1, but their popularity allowed them to have A Day in the Limelight, and Bernard is the only surviving member of the Tailies (Besides Cindy, who became a recurring minor Other.
Despite his placement on the far end of the Sliding Scale of Antagonist Vileness, Martin Keamy enjoyed a substantial following for his short guest appearance due to both Kevin Durand's fabulous performance and for being one of the few LOST villains to be out and out evil.
There's Richard Alpert.
Daniel Faraday developed a huge following, despite limited screen-time.
Frank Lapidus started to become a fan favorite when he was reintroduced in season 5, and was cemented as an Ensemble Dark Horse in season 6.
Joan Holloway. When most non-watchers think of the show, they'll probably remember her first, though she's not the main focus of the show (what with it called Mad Men and all...)
There's John Slattery as RogerSterling. He's practically a punchline in most of the first season, with his skirt-chasing and his outrageously offensive lines. Over the course of the show, he's become a fan-favorite and his relationships with ex-wife Mona, daughter Margaret, current wife Jane, and mistress Joan have been some of the most solid and entertaining on the show.
There's Kiernan Shipka as Sally Draper who is Don's barely there daughter in season one, but as the actress has gotten older and proven her chops, she has gotten more and more screentime and better and better storylines. (Bobby continues to barely exist.)
The Man from U.N.C.L.E.: An example of a successful Retool which made an Ensemble Dark Horse into a Breakout Character: The original premise for the show had Napoleon Solo working alone, with Ilya 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.
Mash: The is probably the original ensemble show, so it's unsurprising that this trope pops up there.
The cross-dressing Corporal Max Klinger was meant to be a one-time joke, but eventually became a regular. He was one of the only four characters to stay through the show's eleven seasons. The other three are Hawkeye, Maj. Houlihan, and Father Mulcahy, who were main characters from the scratch.
Sidney Freedman was almost meant for a one-shot appearance, but was brought back on several occasions.
Colonel Flagg was a very popular and memorable character despite appearing in only 7 episodes.
Merlin: Sir Leon. Despite having only actually turned up to basically provide filler dialogue in 7 episodes over the course of season two and being universally feared to have been killed by dragon flames in the season 2 finale, he returned for season 3 due to the huge fan outrage at his death (screw continuity). After all, when you've married a troll and you need someone to be diplomatically silent about it, who do you call? Sir Leon!
Both Lancelot and Gwaine showed up in an episode, dominated all of their scenes and won the hearts of fangirls everywhere, and caused much rejoicing on their return.
Old Gregg is one of their best known characters, and one of the most often imitated. He only has probably about a page's worth of dialogue, but more than a few fans of the show could recite the entirety of it.
The Hitcher, despite the fact he's a psychopath who would rape Howard behind the counter if he were female.
Barney was originally just in charge of technology, but by season 6, when there was no longer a Lancer, he moved into that role and got numerous scenes designed to spotlight Greg Morris's acting, as well as some romantic storylines.
Willy was just a strongman, and frequently had very little to do as the show got more cerebral. However, when the producers tried to write him out in Season 5, there was a fan revolt, and not only did he stick around but he was given much more to do in Seasons 6 and 7.
During the first season, Martin Landau, though crucial to virtually every episode, was not listed as a regular cast character but rather noted at the end of the cast listing as a "Special Guest Star", as if he was a last minute write-in. After the first season, when Steven Hill left as the team leader to be replaced by Peter Graves as Mr. Phelps, Landau became an official cast member.
Mork and Mindy: Exidor. He started out as the leader of the cult "Friends Of Venus" (the other members of which were figments of his imagination) in the episode "Mork Runs Away". By the end of that episode it was clear the audience loved him almost as much as they loved Mork. He returned in the episode "Mork The Gullible" to rapturous applause, and after that he became a recurring character who was greeted by the audience with cheers throughout all four seasons. Interesting side note: His first two appearances concluded with him being disappointed with Mork for his lack of belief in Exidor's "religions". They eventually ended up as close friends (sometimes they seemedveryclose).
In the first season, everyone thought Frank Oz's main character would be Fozzie Bear. Miss Piggy was a supporting character, so far down the list she didn't even have a consistent performer (Richard Hunt performed her for half the episodes she appeared in). Then Oz was operating her when she was supposed to slap Kermit, and decided to go for a karate chop instead, and something clicked. Oz would later describe her as his only three-dimensional character, saying Fozzie had two dimensions, and Animal zero.
Animal is one of the most popular characters on the show! He has over 700 fans (which is growing) and has been included in more merchandise then the rest of Dr. Teeth and the Electric Mayhem and has been the mascot for the 1998 Snowboarding Team! I certainly love him!
From the worst movie came its most enduring guest character — Torgo, the stammering, swollen-kneed, scruffy-bearded, ratty-clothed, thoroughly creepy yet ineffectual henchman to The Master. Not only does he make numerous appearances and cameos in the host segments over the next couple seasons, he's even given an epic fate as he becomes Torgo the White.
Nip/Tuck: The characters of Kimber and Gina were both originally intended as one-offs, but were promoted to main-character status due to unusually strong performances by their respective actors and resonance with the fandom.
The O.C.: Summer Roberts and Julie Cooper were not originally supposed to be main characters, and their actors were listed as guest stars - even though Rachel Bilson (Summer) was in every episode of the first season, and Melinda Clarke (Julie) was only absent a couple of times. Taylor Townsend might be an even more striking example as she was introduced as a mere minor villain, but ended up not only surviving her arc (and quickly switching over to the side of good) but becoming a main character at the beginning of the following season, and then romantic interest to the main character.
Andy Bernard. Notably, he's the only character to survive the merger from Stamford (other than Jim, of course) and is easily the most prominent character on the show, other than the main four, over all the people who've been there since season 1 (and even Ryan). His actor, Ed Helms, even got featured in the opening credits starting in Season 6.
Erin was initially meant to be a temporary character, only appearing in a few episodes at the end of the 5th season. Her character was well-received by the fans, however, and the producers were impressed with her actress, Ellie Kemper, so she was made a regular starting with season 6.
Unless he's having A Day in the Limelight, Creed Bratton often has only one line per episode, if not less. Doesn't stop him from completely stealing the show in the eyes of the fans whenever he says something.
Belle (who appears in three episodes in season 1 and was upgraded in season two.
Captain Hook who had managed to be promoted to series regular for the second half of season two despite having not yet appeared in any episodes.
She shows up in the last minute of season 3, and doesn't even get a line yet, but the moment that Elsa from Frozen appeared, fans were begging for season 4 to start pronto. Some who were even on the verge of quitting the show were pulled back in just seeing that she was gonna show up.
April started as the snarky intern who was Out of Focus, but hooking up with, ironically, Andy has gotten her more screentime, and recently started to take over some of Leslie's responsibilities
Ben started as the Straight Man, but soon developed a fanbase and became Leslie's Love Interest and the character with the most focus other than Ron and Leslie
Peep Show: Super Hans, Jeremy's techno nutter mate. He's a self centered fantasist but his often hilarious drug addiction and plain coolness have endeared him to many fans (He has a snake, for crying out loud).
Alan Johnson is another candidate. Originally a one-episode character, he became Mark's boss at JLB in the second series, and is one of the more widely-loved characters.
Popular: April Tuna. The sexually aggressive nerd was the weirdest and often best part of the show.
Bulk and Skull. While present in numerous episodes as the comedy relief, on rare occasions they showed a noble side. Most notably and successfully, they led the entire town of Angel Grove in an I Am Spartacus, presenting themselves as Power Rangers.
Bulk himself managed to pop up in Power Rangers Lost Galaxy, a season which had little ties in terms of continuity to the arc that began in the original series and concluded in In Space. Unfortunately, he wasn't paired up with Skull this time around (Skull had overslept, missing the flight to Terra Venture, forcing Bulk and Professor Phenomenus to go on without him), but Wild Force's "Forever Red" reunited the dynamic duo of PR and had the two conversing about the glory days.
Tommy Oliver, Jeebus himself. The franchise wasn't cool until he showed up, he was made into the star, and they brought him back for both the tenth anniversary Reunion Show and an entire encore season. He might be the franchise's original Spotlight-Stealing Squad. And he wasn't even a Red Ranger to begin with.
Almost every season of Power Rangers has one, usually the Sixth Ranger but there are exceptions. First it was Tommy, but once Tommy became the White Ranger and promoted to team leader he still remained popular with the audience.
When it isn't a Sixth Ranger, there's a slight chance that it will be a Green Ranger, given their tendencies to be The Smart Guy, the comic relief character, or a mix of both.
In terms of villains, the Psycho Rangers from In Space were popular enough to have a trope named after them. Out of them, Psycho Pink is actually fairly popular in her own right as she was the least fleshed-out in In Space but received an episode of her own in Lost Galaxy. Not to mention she's a Power Rangers villain that's actually killed a Ranger in battle.
Power Rangers S.P.D. had Bridge, the Green Ranger. When a 15th anniversary reunion show was planned he was given an offscreen promotion to Red Ranger (already having the Mystic Force Green Ranger in play) simply to justify his inclusion.
Power Rangers Operation Overdrive is not one of the most fondly remembered seasons, though even haters really like the Hartford family's butler, Spencer. He serves as not only the Red Ranger's butler, a snarky one at that, but also gives the rangers some words of wisdom that help them whenever they feel down.
Ziggy was among these in Power Rangers RPM, given his comedic persona and his growing relationship with Dr. K.
Megaforce/Super Megaforce has Robo Knight, the robotic Sixth Ranger that is loved thanks to actually having development compared to the other five, and Prince Vekar, the whiny main villain that ends up getting the funniest lines.
Project Runway: Tim Gunn. Many fans maintain that he is 80% of the reason for watching the show.
Psych: Jimmi Simpson became a fan favorite during his brief appearances as Inspector Mary Lightly.
For a villain, Neville is quite likeable. However, given how the character has devolved in a Smug Snake who Took a Level in Jerkass and gets humiliated a lot, "likeable" is a relative term.
Rhoda: Carlton the Doorman was considered such a miniscule off-screen character that an executive producer provided his voice. The character's popularity propelled that producer — Lorenzo Music — into a new career of numerous voice acing roles, including one particular fat cat.
Hooch is the most popular side-character. At first, he was just a background surgeon used to punctuate a joke about J.D. assuming "Hooch" was the name of a black guy in "Turner & Hooch" (for the record, it's the name of the dog in the Tom Hanks movie Turner and Hooch). Towards the end of that episode, as he got more annoyed at people calling him "Hooch", he started displaying a charmingly Ax-Crazy personality as he yelled at them to stop, prompting characters to say the sort-of Catch Phrase "Hooch is crazy!" Thanks to this, he became loved by many fans, especially for his many quotables, despite being in only four episodes in the fourth season. Because of this positive response, creator Bill Lawrence made sure to fit him into the season seven premiere. Unlike other shows, Scrubs is pretty good at making sure they don't overuse an Ensemble Darkhorse.
The Janitor was originally not intended to have anywhere NEAR the amount of screentime he gets. But hey...
"Snoop Dogg intern", who started as a one-line joke, but who stayed around to become "Snoop Dogg resident" and "Snoop Dogg attending".
SCTV: Originally intended to protest against government requirements for "Identifiable Canadian Content", Dave Thomas and Rick Moranis came up with Bob and Doug McKenzie, two half-wit brothers who talk about various things on The Great White North, while they drink beer. However, despite the McKenzie Bros. being incredibly stereotypical characters, this all worked too well and made them really popular with the audience. There was even a movie that was released after the show ended.
Sesame Street: When Kevin Clash began performing Elmo, the character became this. Years later, though, his status as this turned him into a Creator's Pet.
Sherlock: Fans love them some Mycroft Holmes. Oh, yes, they do. (It doesn't hurt that Mycroft is played by Mark Gatiss...)
Molly's pretty popular, especially since she wasn't in the books and was only meant to be in one episode initially. But she's probably the chewiest Chew Toy on TV.
Anthea has had seven lines across the entire series. She still rocks.
The Shield: Had a couple of actors who would qualify as Ensemble Darkhorse: Walt Goggins, CCH Pounder, and Kenny Johnson all found their roles within the series expanded upon as the series progressed, with Walt Goggins's character becoming arguably the second most important character in the show behind Chiklis's character of Vic Mackey.
David Rees Snell as Ronnie Gardocki is also noteworthy considering he started out as little more than an extra, yet quickly garnered a rather large, vocal fanbase and became the defacto "Draco in Leather Pants" on the show, to such an extent that the character was the only non-Vic Strike Team member to survive the finale (granted arrested and facing life in prison/death penalty, which goes along with the Misaimed Fandom of fans taking to the members of the Strike Team in spite of their crimes).
Skins: Several Characters turn out to be this due to there being multiple generations.
Series 1 & 2 has Chris.
Series 3 & 4 has Emily, J.J., Naomi, and Katie for some.
Chloe Sullivan, who, while not taking over the show, has been revealed as one of the "meteor freaks" and thus taken on greater significance in the series' plot.
Lionel Luthor was originally supposed to appear in a few episodes in Season 1, but the fanbase couldn't get enough of the Magnificent You-Know-What, and he was made both a regular and the Big Bad in subsequent seasons.
When Alicia Baker made her debut in the third season, she was intended to be nothing more than the latest "Freak of the Week", just a Stalker with a CrushYandere with the hots for Clark. But her early scenes with Clark in that episode (where they're forced to reveal their super powers to each other, then realize they can relax and be themselves in each other's company) had such a sweetness to them, that "Calicia" became a Fan-Preferred Couple. She returned in the fourth season, complete with a Heel-Face Turn, an Accidental Marriage to Clark...and a tragic death. While she only appeared in 3 episodes, Alicia looms large in Smallville's mythology for 2 reasons: 1) She's one of only 3 women in the show's entire 10 year run whom Clark explicitly stated he loved (the other 2 being Lana and Lois, making Alicia the only one who wasn't from the comics), and 2) In her final episode, she revealed Clark's secret to Chloe, forever altering the "Chlark" dynamic for the show's remaining 5 1/2 seasons.
Soap: The mild-mannered ventriloquist Chuck and his absolute Jerk Ass dummy Bob. Chuck was originally intended to be guilty of killing his brother Peter (the murder mystery around which the first season was based). But Chuck and Bob became so popular the producers realized they couldn't send the characters to jail/mental hospital. So they completely Retooled the ending of the season, changed the murderer to someone else, and Bob got to live out the remainder of the series being a hilariously offensive jerk to everyone in sight.
Space1999: Alan Carter (Nick Tate). He was a prominent supporting character in the first season. However, during the creative shake-up before the second, his character was slated to be axed along with every other supporting role. Fan reaction was so strong that not only was Carter retained, he was given a promotion (from Chief of Recon to pretty much Third-in-Command) and bigger parts in later episodes. Only one other supporting character from the first season (Xenia Merton as Sandra Benes) was retained, but didn't get the expanded role Carter did.
Stargate SG-1: Vala Mal Doran was initially intended to be only a one-shot character for one episode in the middle of season eight, but had such a strong fan reaction that she earned a recurring role in season nine and a regular role in season ten.
Walter Harriman originally had the unenviable task of basically being a verbalexposition device with no name. Started showing up in more and more episodes as a kind of Shout-Out, eventually got a real name (after several years), and even appeared in some of the "Behind the Scenes" specials.
Similarly, Siler, the much-beleaguered gate technician, has gained quite a following from fans who like to see him get electrocuted, knocked out, and tossed back by an explosion every few episodes.
Radek Zelenka was only meant to appear on Stargate Atlantis for one episode, but became a recurring character due to popularity with fans. Some parts of fandom have also embraced one-shot characters Miko and Parrish, as well the recurring character of Major Lorne.
Doctor Carson Beckett would also fit this trope, started off as a recurring character in the first season before being promoted to regular for season two. Killed off in the third season. Only to be brought back in the fourth season after fan outrage caused a re-think.
Pretty much the same thing has happened to the gate technician, who acquired a fan nickname that became canon (Chuck), and has gradually gained personality from season two onwards.
Sorry, not true. Whilst the fans did have a name for him, the Chucknician, Chuck gained his name in canon by actor Torri Higginson accidentally referring to actor Chuck Campbell by his name during a take which the producers decided to use. At least that's what Chuck Campbell said whilst on stage at a convention.
Todd the Wraith is also up there in fan appreciation, probably due to being the only Wraith in the Pegasus galaxy with a sense of humor and utterly stealing any scene he's in. Originally just a random Wraith that cooperated with Sheppard to escape Genii confinement, he went on to aid Atlantis a few times against the Asurans and became a recurring character.
Rodney McKay was only brought in for a few episodes of Stargate SG-1 but proved popular enough to be a major character in Stargate Atlantis with whole episodes dedicated to his character.
Stargate Universe: Has currently Greer and The Destiny. Greer is obviously well liked, being an utterly cool pyromaniac Bad Ass and as for Destiny... just see the WMG!
Garak, perhaps the Darkhorse of Star Trek: Deep Space Nine, who started out as a minor character with a mysterious past and became a crucial part of the later seasons' intrigues. An exiled patriot, a tailor, a spy, a gardener, a soldier, The Chessmaster, and practically a series regular by the end. He appears only in 33 episodes out of 173. Andrew Robinson was even offered a promotion to main cast member (with his name in the opening titles) for the final season, but declined.
Martok was envisioned as a one-show guest character, but the producers were impressed enough that they brought him back for one more episode where he was revealed as a changeling impostor, who was killed. But again, they liked the actor so much that the real Martok turned up alive and well in a Dominion prison camp and promptly rescued. From that point, General Martok adopts Worf as part of the House of Martok and rises to become Chancellor of the Klingon High Command.
Morn began as an extra who could be seen doing nothing at Quark's Bar. He never even talked. But he became so popular that not only did the character get mentioned constantly (usually said to be a huge talker) and even got an entire episode dedicated to him, "Who Mourns for Morn?"
Weyoun. In his original appearance on the show he was killed. The entire concept of the Vorta being clones was made just to bring him back. It helps when you're played by Jeffrey Combs .
Recurring Andorian character Shran, who, had there been a 5th season, would have been promoted to main character status due largely to his popularity. It helped that he was played by Trek Veteran actor Jeffrey Combs.
Lt. Barclay started as a minor character in A Day in the Limelight episode, but then became something of a Darkhorse as the series progressed. He appeared approximately once per season.
Miles O'Brien. He started as a nameless pilot with a red uniform, then appeared as a security guard, and finally the transporter chief. Eventually he was given a last name, then a first, and then an episode centered around him. He later became a major character in Star Trek: Deep Space Nine.
Worf. The character was an afterthought, not even appearing in the original series bible. Michael Dorn was cast in the pilot for what was conceived as a recurring role as Worf, but he played the part so well that he was immediately, before the show even aired, bumped up to regular status. Dorn played the character for 11 seasons (7 of TNG and 4 of Deep Space Nine) and 4 films, becoming, after Captain Picard the most recognizable character in the modern incarnation of Trek.
Mr. Pointy-Ears himself, Commander Spock. He was originally supposed to be a side character to CaptainKirk, but fan interest quickly exploded and he was soon recognized as the most fan-loved character in the original incarnation of Trek - and maybe the entire franchise. The dynamic duo of the golden-haired Captain and the pointy-eared, dark Vulcan captured fans' hearts and fired their imaginations, quite literally giving birth to Slash Fic in the earliest recorded case of Ho Yay in modern media and becoming a staple of the original series. Interesting to note is that the studio executives thought that Spock would be the scariest character on the show. Spock had an even smaller role in the original pilot, "The Cage", but ended up being the only character to survive the recasting. More specifically, Roddenberry was given the choice to keep either Spock or the female first officer played by Majel Barrett.
A case can be made for the holographic Doctor. He had some of the most consistent writing, compared to, say, Janeway (whose actress had serious trouble figuring out her motivations episode by episode), and Robert Picardo actually enjoyed his role while most of the others were at best staying on for the cheque.
Seven of Nine. Which is impressive, since at first glance, she looks like Ms. Fanservice incarnate — but lucky for her character, Jeri Ryan can act.
It has many popular characters, but in later years, Marika "Jasmine" Reimon from Tokusou Sentai Dekaranger is deeply loved by the fandom. One of most beautiful and snarky Sentai heroes, she's proved to be very popular.
Amongst the old school series, there's Gai Yuuki from Choujin Sentai Jetman. Despite being a Jerkass ranger, he's that Badass and well loved that eventually he evolves into a Jerk with a Heart of Gold and eventually deemed Too Cool to Live and got killed off in the epilogue. There's a manga about him being replaced, but he still proved extremely popular and remains as one of the most favored Sentai Black Rangers EVER. So much that... he's the Jetman representative for Kaizoku Sentai Gokaiger, coming Back from the Dead just for that one episode before returning to the land of the dead.
Kaizoku Sentai Gokaiger has one darkhorse, Luka Millfy, as if succeeding Jasmine in darkhorse department. She has a really antiheroic attitude for a girl, atypical of even the most snarkiest Sentai girl ever, as well as her looks and her tough girl attitude, it helps a lot. note We'd like to call Captain Marvelous another darkhorse, but he's the main character, so he's just a really shiny awesome horse, not a darkhorse
Female blue rangers are usually really well-received. Megumi Misaki, Ako Hayasaka and Urara Ozu receive quite a lot of warm welcome from the fans. However, the one who's truly the female blue ranger Ensemble Dark Horse of epic proportions would be Nanami Nono who combines her gorgeous look, cheery personality and some Crowning Moment Of Awesome that makes her near-guaranteed that she'd be one of the favorite blue rangers ever amongst fans (although sometimes there are some men that could surpass her at times). Hell, Nanami herself appeared in not just one, but TWO (or three) anniversary-dedicated episodes (Boukenger vs Super Sentai or Kaizoku Sentai Gokaiger during the Hurricaneger tribute episode)
There were a lot of darkhorses in several other series though. Back in Dai Sentai Goggle Five, the darkhorse seems to go to either Kanpei Kuroda/Goggle Black (for having a lot of Badass stunts and being played by Junichi Haruta), and Miki Momozono/Goggle Pink (probably due to her Plucky Girl-ness), and at Kagaku Sentai Dynaman, the Black-Pink combination (Ryuu Hoshikawa/Dyna Black and Rei Tachibana/Dyna Pink) become the darkhorses as well (due to, again, Hoshikawa being played by Junichi Haruta, and Rei being played by Sayoko Hagiwara, both of them still resume acting afterwards). At Choudenshi Bioman, it's not the heroes who's the darkhorse, but the villain Bio Hunter Silva who attains this status, despite his short screentime, he instilled fear to many audience with the possibility of how any of the Bioman can die by him, and his reappearance in Super Hero Taisen is received very warmly. And then, jumping on to Choushinsei Flashman, Sara/Yellow Flash also gains a lot of attention due to her borderline Ms. Fanservice outfit and having a lot of drama to go at the end of the series being revealed as Tokimura's daughter. And later on, in Ninja Sentai Kakuranger, Tsuruhime/Ninja White is the one to attain this status, guaranteeing her to appear in many 'Top heroine list', and even could be attaining top spot against modern day heroines, probably due to her being the first technical female leader of the team.
Tommy's Japanese counterpart Burai, aka Burai/Dragon Ranger qualifies too. His On Borrowed Time situation keeps his screentime very limited and he doesn't appear in the opening at all, yet he attains so much popularity for being a conflicted Badass and having strong drama with his brother Geki/Tyranno Ranger that when he died, fans protested to bring him back (without much success).
Despite the fact that Tokumei Sentai Go-Busters didn't do well at all in Japan, Enter, Masato Jin and Beet J Stag are very popular. Jin and J are well liked for their personalities and humorous interaction, and Enter is well liked for being a genuinely threatening and smart villain who just loves doing the bad things he does.
In terms of season, Samurai Sentai Shinkenger is hugely popular. While some may argue it has the deck unfairly stacked in its favor (it was the replacement 18th season of Power Rangers to many fans), it attracted much love. The fans who came from Power Rangers stuck with Super Sentai because of its well told stories, hamminess, and for not being afraid to break the standard Sentai mold.
Other popular seasons include Choujin Sentai Jetman (mainly because of the aforementioned Gai, but also because of its compelling story and well-thought out characters like Radiguet and Grey (and how much of a S.O.B. Radiguet is)), Juken Sentai Gekiranger (because of Gou/Geki Violet, and the whole Super Sentai x Wuxia concept) and series like Gekisou Sentai Carranger (because of the sheer LULZ it generates).
One season Toei seems to love is Choujuu Sentai Liveman. This season, by far, is one of the darkest seasons as well as the first season with two giant robots combining, gets small nods throughout the series (Go-Onger's core team had the same colors as the heroes of Liveman in the same two sets- Red, Blue, Yellow and Black, Green, and Hurricanger has been compared to Liveman often), and shows up a lot in the anniversary seasons (Yuusuke/Red Falcon is a member of the Dream Sentai team in Gaoranger vs Super Sentai and is, barring Soukichi Banba/Big One, the only hero from before the 20th anniversary to show up. Also, Liveman is one of only three shows before Dairanger to get a tribute episode in Gokaiger, along with Battle Fever and Jetman.)
Castiel was only supposed to appear in six episodes in Season 4, but he made such a good impression on the fans that they've made him a regular for Season 5. The fact that he's extremely attractive probably doesn't do his popularity any harm, either. The actor that plays him, Misha Collins, got A Day in the Limelight with the Season 4 episode "The Rapture", though not playing Castiel in most of it. To reiterate: Castiel is one of the most renowned examples in the TV genre. He's so prominent that people outside of the fandom will recognize him and not the Winchesters, the show's actual stars. In fact, new fans are always surprised when they first watch the series and find that not only does Castiel not come in until Season 4, but he doesn't even appear in half the episodes.
Death. Despite his very limited appearances, he has very devoted fans. Might have something to do with his promise to reap God.
The demon Crowley has become this, especially after his involvement in episode 5x20. It probably helps that he's played by Mark Sheppard.
Gabriel is also this, having both a large number of fans (he even has his own Big Bang on Livejournal) and a lot of people who want to see him again even after he was Killed Off for Real by his brother.
Meg is also popular to many fans.
Bela, the latter the subject of a petition to bring her back to the show, despite having been ripped apart by hellhounds.
Hendricksen and Gordon Walker both had quite a small number of appearances despite being quite popular.
One first-season episode had Latka marrying a call girl so he could remain in the US. The "minister" who presided over the ceremony, Reverend Jim Ignatowski, proved such a hit that he became a cast regular the following season.
Jeff, the guy who stood next to Louie in the "box", was originally just an extra, but over time he was given lines, some of them even significant. During the final season, he got A Day in the Limelight.
StilesStilinski. While the intended hot and popular characters are the Mr. Fanservice werewolves Scott, Derek, and Jackson, it's scrawny, human, Muggle Best Friend Stiles who's gotten a lot of fans hot under the collar and begging for more of him in upcoming episodes.
IsaacLahey also has had this effect for quite a few fans.
Danny Mahealani. He's Jackson's Straight Gay best friend, has only had about a dozen lines and has only had a real influence on the actual plot maybe twice, but you'd be hard pressed to find a fan of the show who doesn't love him.
That '70s Show: Eric (played by Topher Grace) was the main character, but Kelso (played by Ashton Kutcher) quickly became the most popular one. This reportedly led to some bad blood between the actors, since Kutcher began getting the most lines in what was supposed to be Grace's show.
Forget Kelso, the true Ensemble Darkhorse is RedForman. Red is easily the most quotable character in the series and his no-nonsense attitude towards his son's (and everyone else's) antics have won him a legion of fans.
The Thick of It: Jamie has only been in three episodes, but is one of the most popular characters.
Top Gear: The Stig, originally a Suspiciously Similar Substitute of the first Stig and only around to drive the cars around the track, became a surprisingly popular character, to the point where he participated in some of the other stunts Top Gear did (all while wearing his trademark helmet and jumpsuit).
Torchwood: Ianto Jones was originally just the Torchwood Three secretary and, with a couple of noted exceptions, had one or two lines an episode. However, the fans loved him and he became significantly more prominent in series 2 and 3. He was not even meant to survive series 1. Then, he was planned to be the zombie of Torchwood in series two and suffer Owen's fate.
Camp Gay character Lafayette Reynolds was supposed to get killed at the first season ending. Up to season 3, he's still alive because fandom liked him too much. His level of Ensemble Dark Horse is so impressive that despite his programmed death in the books he has his own gay love interest, has become part of the main cast AND he'll be part of the major plot in season 4. Not to mention that at the end of season 3 it has been mentioned that he has great magical potential inside him.
Pam is very popular among fans; she has become part of the main cast and is getting much more screen-time. However this is not as impressive as Lafayette's case because she becomes a major character in the books too.
Jessica has a very big fanbase despite not being in the books at all.
Despite only appearing in a handful of episodes, being alive in only a few of them, Godric is very popular among the fans. Probably because he is a total Badass.
Anne Boleyn is often depicted as a villain, and in the beginning of the series, most fans thought she was quite the bitch. Though as the series progressed she became one of the most popular characters in the entire show and a lot of fans considered abandoning the show after Natalie Dormer's departure.
Thomas Cromwell. In nearly every single telling of what happened during the reign of Henry VIII, Cromwell is depicted as a heartless, evil villain. James Frain's Cromwell however is a well-liked character among the fans, because of the actor's ability to give him heart and soul.
Towards the end a lot of people were most invested in Mary, largely due to Sarah Bolger's strong performance. The fact that Mary wasn't villainized (as she often is in stories about Queen Elizabeth) also helped.
U-Pick Live: To a much, much lesser extent of the trope, "Pick Boy", one of Nickelodeon's afternoon in-between-shows programs. Pick Boy is a egotistical yet naive "Super Hero", whose main jobsuper-power is to "pick" people from the studio audience to participate in games. Pick Boy became a prominent character, and target of many polls at nick.com, such as "What kind of hairstyle should Pick Boy have?" Despite his popularity, Pick Boy was kicked-off the show when he was voted the most in a poll asking "Who should be kicked off from the show forever?", but was brought back next season. Even after U-pick Live ended, his character is constantly used, and has made many appearances in ads, in the 2006 and 2007 Nickelodeon Kids' Choice Awards, and even the show that succeeded U-pick Live, Me TV.
President Bartlet was originally supposed to be referred to often but rarely actually seen, but everyone liked his one scene in the pilot so much that he was made a series regular. More than that, he was made the lead character of the show. Not just the focus, but the protagonist. Probably the largest Darkhorse bump ever.
Josh Lyman is another. While he was fairly prominent from the get-go, by the third season he's definitely the secondary main character after Bartlet, and by the final season the rest of the original cast has become supporting and it's essentially The Josh Lyman Show. It helped that he was the DeadpanSnarkiest of a show filled with Deadpan Snarkers. Josh also had great help (as usual for him) from his assistant, Donna Moss, who was a "guest star" in every single episode of the first season before joining him in the opening credits from the second season on. The chemistry between the actors Bradley Whitford and Janel Maloney was off the charts, so much so that Entertainment Weekly called them the "IT couple" in that first year. Donna would be a major player for the rest of the series' run, culminating in the final season turning up the Will They or Won't They?Up to Eleven untill They Do.
Jeff Davis, who was the youngest comedian on the show, proved to be very popular with fans, despite not appearing in many episodes. His popularity led to him being the first fourth-seater in the 2013 CW revival
Speaking of the CW revival, recurring fourth-seater Gary Anthony Williams has gained a following among fans as well.
The Wire: So great is the popularity of Badass Longcoat-wearing gunslinger Omar Little, it can come as a shock to remember that he didn't even get a credit in the opening sequence until the third season. Of great credit to the show's writers is that they didn't attempt to retool the show around him, and in fact treat him as simply a gangster like any other. But that doesn't mean they never made any reference to his popularity either: he is worshipped by many of the homeless street kids, illustrated in a scene during the third season where Bunk spots a group of kids squabbling over who gets to "be Omar" in a roleplaying game. A case of a character being an Ensemble Dark Horse even amongst the Ensemble itself? Omar's popularity also undoubtedly contributed to the brutality and suddenness of his death. The show's creators prided themselves on never succumbing to narrative conventions or audience expectations, and their refusal to have their most popular character go out in a blaze of glory - he gets shot in the head, midway through the final season, by a ten year old Hopper whilst out grocery shopping - could be interpreted as them refusing to compromise this. Another point they could be making with Omar's death: Legends about people like Omar will always carry on long after their deaths. Despite Omar being gunned down by a kid, despite the pathologist being so uncaring that he even puts Omar in the wrong body bag, the final time Omar is mentioned in the series is in the finale: a group of kids exchange different versions of how they'd heard Omar died, each story far grander than what really happened.
Skinner. He appeared in only one episode in season one but his role was getting substantially bigger as the show went on. For some people, him just showing up in the second movie was the best thing about it because the series finale implied he might have been offed.
Eugene Victor Tooms distinguished himself as one of the series most popular Monster of the Week, even being one of the few to appear in more than one episode.
Mrs Scully. Sheila Larken's performance was always a joy to watch. It was pity that Mrs Scully usually showed up only for a family crisis and thus was seen to suffer a lot.
That sweet Horny Scientist of The Lab Rat Agent Pendrell. Amazing that he managed to become a fans' favourite as his role was really minor and he was killed off way too soon.
The Young Ones: This show is one of the purest examples of an Ensemble Cast, being that no one character was ever featured over the others throughout its run and everyone got a good chance to shine. Still, ask any Brit old enough to know to quickly name one of the lads, and nine times out of ten they’ll say "Vyvyan" first. Then again, when you look at the pilot, Vyv is the last lad to be introduced and the one given the most dramatic entrance, indicating that the creators probably figured he’d attain this status anyway.