Two-Tone is without a doubt the most popular Dalmatian in the series, in fact so popular that fans felt she deserved to be the fourth main pup instead of Spot. She's mainly popular for her unique spot pattern and tomboyish personality. She usually appears in the background in most episodes but has had some speaking parts in a few episodes. Her claim to fame was the episode, "Love 'Em and Flea 'Em" where she played a major role and was in a romantic relationship with Lucky. Also, being voiced by Tara Strong just adds to her popularity.
In a similar fashion, Patch has become quite popular despite looking much differently than he does in the movies. Most fans prefer his movie design because it's cuter, but others prefer his series look as it is closer to Dodie Smith's depiction of him.
Tripod has a fairly large fan base, namely for being a Handicapped Badass. A lot of fans like to pair him up with Cadpig (the general reason being that the two would likely bond over their physical shortcomings and insecurities interpreted by the audience), despite the fact that neither of them conversed with each other at any point in the series.
Mooch, the farm bully, both loved and hated by fans alike. Another character that fans like to pair up with Cadpig since they were both in a romantic relationship in the episode, "My Fair Moochie" (even though Cadpig was only pretending to be his girlfriend.)
Wizzer and Dipstick both seem to have a lot of fans. Often depicted as bullies since they're usually seen hanging out with Mooch. They're also well known for having big parts in the live action films and the video games.
As a one shot character, Blaze has also been a subject of Cadpig shipping, except in this case, Cadpig did show some genuine interest to him (such as calling him a "hunk" and batting her eyelids to him). It also helps that Blaze is a Nice Guy with amusing acting talent.
Ivy, Cruella's niece, is the single most popular antagonist, even though she only appeared in three episodes.
Amber, Anita's niece, has gotten quite a cult following for a one shot.
The Dalmatians' hero, Thunderbolt, has an impressive fan base of his own outside of the universe. Some have even gone as far to make fan made girlfriends and wives for him.
Even though he was only in 101 Dalmatians II: Patch's London Adventure and not in the series, Thunder's sidekick, Lil' Lightning has also gained a cult following.
Jewel is a Dalmatian pup who did not appear once in the series, but she has appeared a lot in fan art with a particular fan depiction. Fans speculate that if Jewel was a character in the series, she'd have a valley-girl-like personality.
Pretty much every Dalmatian pup with a name who didn't appear in the series has a fan base of some kind (e.g. Penny, Pepper, Freckles, Fidget, etc.), Penny especially, considering she had a fairly big role in the animated movie and was originally intended to be one of the main pups in the series (in some countries, she's in place of Cadpig).
Marceline the Vampire Queen. She was already intended to be a recurring character, but after her first appearance she was already a fan favorite. She seems to show up more often now as a result.
Lumpy Space Princess. "Bunch of smooth poseurs."
Marshall Lee, the gender flipped version of Marceline from "Fiona and Cake", only showed up for a few seconds in the episode and had no speaking lines. For well over a year since his appearance, he has skyrocketed into possibly the most popular character on the show.
Flame Princess was more popular than the alternate love interest for Finn despite only having 15 episode appearances.
Of its darker sister show Sonic SatAM, there is Bunnie Rabbot. She arguably had the least spotlight of the main Freedom Fighters, acting as a bit player at best in every appearance, though is popular for her cyborg powers and is one of the few alternate media Sonic characters not to gain much distain.
Principal Lewis, for being the most outrageous character other than Roger.
Greg Corbin and Terry Bates, the gay local news anchors.
Michelle, the worst lawyer in the afterlife, enough to cameo in the Series Fauxnale "Rapture's Delight".
Many people remember the Majestic, but that's mostly because of its theme song.
In The Animals of Farthing Wood the most popular character is Bold, Fox and Vixen's oldest son. After a reckless attempt on Scarface, he leaves White Deer Park to live his own life. He learns the hard way how to provide for himself, gaining a partially blind eye and a bullet in the leg. He struggles to earn the love of a vixen (Whisper), who only becomes his mate after finding out his father is a figure of local legend. He swallows his pride and takes her back to White Deer Park so she can have their cubs in safety. The injury in his leg ultimately kills him, but not before he knows his father is proud of him. And he only appeared in eight episodes of a three season series.
There's also Jun, which the creators even commented on. Likely why she reappears in the Grand Finale.
Both Suki and Jet were meant to be one offs as well. Fan demand brought both of them back in substantial parts and Suki even joining the gAang.
Jin as well. She appeared in one episode in Book 2, where she went on a date with Zuko, and had a cameo when Ba Sing Se fell. You wouldn't have any trouble finding fan art for her, and there is a fairly strong following for the Jin/Zuko ship.
Although it's not exactly as much "popular" as "well-remembered", Koh. He barely had any plot-importance even the two times you see him (one was a flashback), but he's was just so damn creepy that he's become a frequenter of fanworks and Epileptic Trees all the way up to the end of the show.
Ultron first appeared in episode 5 as an innocent prison guard. As the first season progressed, he gradually entered the foreground while obtaining the knowledge that would turn him into the Killer Robot Marvel fans know and love. He finally turned against the Avengers in episodes 22 and 23, performing such feats as possessing Iron Man's armor, seemingly killing Thor, and using stolen nuclear missile codes to try and wipe out all "flawed" life on Earth. The next season, he unexpectedly came back, upgraded his body, and replaced most of the Avengers with ruthless robot duplicates. Even though Ultron ultimately only acted evil in two episodes each season, an unofficial poll deemed him the most popular villain by a longshot.
Mockingbird has become regarded by fans as a fun character, despite only making three appearances under that alias. There's a significantly vocal group who wished she became part of the New Avengers. The choice to give her a bigger role in the Secret Invasion arc was actually thanks to Chris Yost noticing the positive reception she got.
Doctor Doom won fans over even before his first episode premiered. His actual feats didn't disappoint either; he overcame six Avengers and the entire Fantastic Four in a battle, and found a Skrull among them even before they could. Doom did another guest spot 10 episodes later, in which he helped Iron Man create a weapon to expose disguised Skrulls. Doom explained that he spent so much time trying to prevent the Skrulls from taking over the world so that eventually, he could take it over instead.
As Iron Fist and Luke Cage beat up thugs, cracked dry wit, and helped track down the thief of Hank Pym's Ant-Man suit, the viewers wished they could watch these Heroes For Hire kick even more ass in their own show.
The series was also responsible for the rise in the popularity of villains like the Mad Hatter, Clayface, and Mr. Freeze, who until then were practically obscure to people who weren't into the comics. Special mention to Mr. Freeze's case, as his backstory in the animated show was so well-written that it won an Emmy. It was also adapted into the comic book series and turned him into another memorable villain of Batman.
"Thriftie", an unnamed character with only one minute of screen time.
Roxy Rocket only appeared in two episodes (one of which was only a cameo) and yet has gained a strong enough following to become a Canon Immigrant.
Baby Doll is arguably the most popular of the BTAS exclusive villains.
The Music Meister, despite only being the main villain of a single episode. Though, the fact he's voiced by Neil Patrick Harris probably helps. After the episode was out there was tons of fanart, and demands that he be incorporated into the DCU within minutes. The demand for the soundtrack was so high that they announced plans to release a soundtrack about two or three days after the episode leaked onto the Internet, and said that if the show had a third season he would be back. (Unfortunately, he wasn't.)
Ben 10: Ben is technically an army of Ensemble Darkhorses, being a shapeshifter who can assume various alien forms, across the series he gets a lot of forms that are heavily popular amongst the fans even when not playing an especially major point in the plot or even having limited appearance:
Aside from the aliens forms, several minor characters and villains in both series have become Ensemble Darkhorses. Those include Charmcaster, Lucky Girl (though this is actually Gwen is disguise), Kevin (who later was upgraded into a main villain and then a hero), Albedo, Darkstar... and Paradox is probably as popular as Rath himself.
Articguana and Buzzshock, two of Ben 10,000's aliens, became rather popular, with fans wanting to see them be unlocked. The fans got their wish with Articguana, who finally got used in Omniverse, and Buzzshock a couple of seasons later.
Simon Belmont, despite being rather Out of Character, has gained some popularity if only because he seems to be the only main protagonist with any sort of personality. Being a constant Butt Monkey probably helps this, too.
Sector Z! After five minutes of screentime in the movie they won the hearts of a sizeable portion of the fanbase with their badassery. It probably helps that they were the Delightful Children from Down the Lane before they were Brainwashed and Crazy.
Fred the Barber Half the show's references on this site are about him. He was the least dangerous but most memorable villain.
Di Lung. To the point where he would appear more frequently during the later seasons.
Katz is another example. His popularity may be the reason he appears more often than other villain on the show.
King Ramses' is quite popular despite the fact that many find his episode to be the scariest. In fact, the reason as to why most people assume he is scary is possibly an excuse to make him seem more popular.
Freakshow, the only human character who could truly be considered an antagonist. He only appeared as the Big Bad of one episode, and the third TV movie, but is one of the more popular villains, with the general consensus being that They Wasted a Perfectly Good Character.
Stacy Rowe is possibly the best example, as her popularity with the fandom was likely the reason why she got an actual, honest-to-goodness recurring subplot in season five, which, for any Lawndale High character not named Daria, Jane, or Quinn, was really quite a feat.
Daria's Cool Aunt Amy, despite only showing up in three episodes (one of which was a cameo).
The shorts "Dial M for Monkey" and "The Justice Friends" are just as popular if not more so than the main Dexter shorts.
Within the main cartoon itself, Dexter's Mom and Lisa the babysitter. Guesswhy.
The Touchy-Feely Neighbor Lady is widely remembered, despite only appearing in one episode.
Mandark's abrasive younger sister and Dee Dee's rival-turned-friend Lalavava only appeared in one episode, yet has a substantial amount of fanart, enough to have gained her own category in some fandom groups for the show.
Lee-Lee, one of Dee-Dee's friends, is one of these
Douglas Mordecai, one of Dexter's friends, despite being a borderline Satellite Character in canon. Maybe just because he was the only character Dexter could talk to like a human being.
Val Halen has a ton of fangirls.
The Puppet Pals. Some people wished they had their own Spin-Off.
Scrooge McDuck started as a supporting character in Donald's comics, but eventually became popular enough to be spun off into his own series. Scrooge is much more popular than Donald in Finland and France, where the magazine devoted to Carl Barks' universe is called "Picsou Magazine". (Picsou being the French name for Scrooge). DuckTalesputs Donald on a boat in the first episode, reducing a character that was always going along with Scrooge on his adventures to a guest character. Considering Scrooge started as nothing more than a caricature of thrift in a propaganda cartoon during World War II, that's one escalation.
Doug: Roger Klotz and his gang, the chief antagonists. Also, Doug's older sister Judy Funnie.
Glittering Goldie, who appeared in only one Carl Barks story, appeared in several episodes.
Launchpad McQuack, who later became Darkwing Ducks sidekick. Look on the internet; there are plenty of people talking about how sweet he is or how funny he is.
Don Rosa, who made it quite clear that he wasn't very found of the series portrayal of the Donald Duck mythos. Once mentioned that Launchpad was one of the exceptions and that he would really have loved to use him in a mainstream Donald Duck story if Egmont had let him.
Gizmoduck, plain and simple. He turned an accountant into the most powerful array of mechanical components and accessories who is a bodyguard and security guard for Scrooge, as well as a super hero to Duckburg and even went on to help Darkwing Duck protect St. Canard.
Marie, the blue-haired Kanker. She has a much larger fanbase than any other girl in the show and is often considered to be the most attractive one, despite not being intentionally designed as such. Pairing her with the object of her affections/psychosis, Edd, is probably the most popular 'ship in the fandom. Her sisters get this to a lesser degree, as does Rolf.
Plank. Fans who have made stories focusing on that piece of wood make him out to be secretly an evil genius due to apparently being sentient and being only able to "talk" to Jonny. Also, there's a plushie of him◊.
Ms. Doombringer, a highly competent fairy hunter that appeared in just one episode but has people asking for more.
Molly, a girl that showed up for one episode but already has shippers pairing her and Timmy together.
Remy Buxaplenty to some degree - he appeared in one episode to challenge and lose to Timmy in a duel, and disappeared after that. All the same he had a fairly significant following and a tendency to be shipped with Trixie Tang - who wasn't even in the episode he was in - and years later was brought back for a small handful of new episodes.
Dark Laser. Initially, he was a one off villain. Then he made an appearance in the fifth season as a continuity nod, but by the sixth season, he'd become a semi-regular, even having a major part in the season finale.
Anti-Cosmo is quite popular among the fandom, especially the idea of Timmy being his godchild. No wonder Foop is The Scrappy.
Numerous characters in Family Guy. Includes Ernie the Giant Chicken, Death, the Evil Monkey, Ollie Williams, Seamus, Herbert, Greased-up Deaf Guy, and Bruce (the Performance Artist that has "Oh no!" as a Catch Phrase) are all popular among fans. They were also one time characters before cancellation but due to their popularity they became Recurring Extras soon after.
Scruffy was a popular character during the interim when Futurama was not producing new episodes. He only made a few appearances in the original run, and was mostly noteworthy for his extreme apathy. Since the revival, Scruffy has had a much larger role, frequently appearing alongside the main cast, almost solely due to his popularity on the Internet.
Odie was also one, due to his cuteness factor and being more sympathetic than Garfield.
Wade Duck from the U.S. Acres side is one of these. His cowardice was so over the top, you couldn't help but laugh (believing he would go to prison for removing a mattress tag, that staplers spy on important documents and will take over the world, that football players in a huddle during a game, no less are talking about him behind his back, etc.) Bo was one as well, to a lesser extent given his laid-back attitude and despite the fact that he was not seen nearly as often as his twin sister, Lanolin.
Generator Rex has Breach, who gained a bunch of fans due to a combination of some Foe Yay with the main character, and being attractive while being delightfully creepy at the same time. It got to the point where the fandom had a collective freakout when she seemed to be killed off, then rejoiced when it turned out she was fine.
Max's cousin Debbie only appears in one episode ("Leader of the Pack"), but is almost as popular as the main cast.
Mrs. Pennypacker also only appears in one episode ("Date with Destiny") and is also very popular, though not quite as popular as Debbie.
There's Peg, who, while technically a member of the main cast, was mostly used as a supporting character and only had a few limelight episodes. Judging by volume of fanart, you'd think she was the main character.
One of the Trekkies in A Goofy Movie has reached meme status despite having no name, no plot relevance, and only two lines (spoken by different actors) in the entire movie.
Bigfoot. Plenty of people testify that his short suite of scenes were the funniest part of the movie.
Bobby Zimmeruski was so popular in the first movie—despite being in only six scenes (and only doing something notable in five of them) and having barely any plot relevance—that he became a main character in the second movie. The other new important movie characters meanwhile, fell victim to Chuck Cunningham Syndrome.
Beret Girl in An Extremely Goofy Movie is extremely popular, despite having no name and only appearing in four scenes, because of her appearance, her interesting personality, and the fact that she's the final love interest for PJ.
Giffany, the yandereAI from "Soos and the Real Girl," gained a lot of fanart within a mere week after her debut.
While he had a handful of fans beforehand, Robbie V is quickly becoming this since the events in "The Love God" revealed him to be just a goofy, maladjusted, insecure teenager with very... interesting parents.
Charity Bazaar from Histeria! is stated to be the best character on the show by even people who hated it. In fact, the whole Kid Chorus seems to be more popular than the hosts (with the exception of Miss Information).
Gene Khan in Iron Man: Armored Adventures. Yes, even after the season one finale and what he did during it. He is the Ensemble Darkhorse of the fandom. Two of the longest Fan Fics in the IMAA fandom focus on him. Fans like pairing him up with Tony and Pepper. The fan forums' Epileptic Trees tend to center around him quite often. His mother also gets discussed/written about quite a bit given that we only see her in a single flashback once.
Tak. Despite appearing in one two-part episode and having a vocal cameo in another, the amount of Fan-Art depicting her can easily confuse new fans into thinking she's part of the main cast. She had one planned reappearance before the cancellation of the series, but it's certain her popularity would have resulted in many more. Given that she was the first competent Irken we had ever seen, and made a great female foil for Zim, it's easy to see why.
THE MOOSE! Seriously. The Moose has its own fanbase just for being so damn impressive.
Minimoose, who only appeared in one episode before the show got cancelled, makes regular appearances in fanart and fanfic, usually as the Woodstock to GIR's Snoopy. He's just so darn cute...
Jade's (technical) Superpowered Evil Side, the Queen of the Shadowkhan. Despite only showing up in one episode in Season 2 (and getting a brief mention by Tarakudo in Season 4), there are quite a variety of stories in which she regains the powers and/or personality of the Queen. (See Project Dark Jade.)
The Demon Sorcerers also count, though not to the same extent as Queen Jade because they got more screentime. They were a Big Bad Ensemble in the second season, but were never heard from again after their defeat. The fandom loves these guys, who were each given decent and distinct personalities despite only really getting one episode each, plus a few group appearances.
In particular Hsi Wu, the sky demon. Somehow the one episode he has a significant role in managed to make him a fan favorite for shipping with Jade. Or else everyone just hates Paco that much...
From Jem we have Stormer in the Misfits. Her popularity is fueled by her being the Token Good Teammate of the Misfits, albeit she is capable of having a nasty attitude from time to time. She's the overall creative force behind her band but she never gets credit for it, and usually winds up helping (secretly) the Holograms when she absolutely feels she needs to. Stormer has also been the only Misfit to be offered a place in the Holograms, but she turned it down because she felt the Misfits needed her more. Word of God has it Stormer's first loyalty is always going to be the Misfits regardless, which really only makes her more likeable; she sticks by her friends.
Green Arrow. This version of the character adds a sarcastic side at the series and many like his Odd Friendship with Supergirl and the love story with Black Canary.
Black Canary herself that is one of the most Fanservice character in the series, but also a skilled woman with a strong temper.
Amanda Waller, as voiced by CCH Pounder was regarded by some fans to be a highly well developed and interesting character who, as a mere human, outwitted and kept the league in check. She was already well known in the comics but the series version of her brought a new legion of fans.
Giganta. Fans find this version of character cute and adorable. She has also Ship Tease with the fan-favorite Flash in the last episode.
Galatea who is an evil version of Supergirl, but her appearances are based on Power Girl with the white costume and the famous Cleavage Window. Beyond this, his fight with Supergirl is one of favorite moments by fan.
Word of God is that Dr. Drakken and Shego were originally just supposed to be a part of the show's rotating cast of villains, but became so popular with the fans that they were used more and more often until they became regulars. They eventually hit a point where they not only were the most fleshed out villains in the series but also began having recurring bouts of Enemy Mine, and then came the Grand Finale of the show where they actually become key players in saving the day.
Electronique was rather popular with fans, more than you'd expect from her single appearance. But then, she was a Baroness and Gadgeteer Genius with electrical powers, a short temper and a hammy accent from somevhere in zer centre of Europe, so she was a memorably entertaining character, and as a villain in Shego's hometown she had a connection to the fandom's favourite character.
Chomper from The Land Before Time 2 was so popular with fans and critics that he gave him a second Guest Star Party Member role in The Land Before Time 5, this time with a speaking role. This increased his popularity to the point that fans began perennially requesting for him to appear in the sequels that came afterwards. While he never made another appearance in any of the movies, when the TV series was released Chomper was not only brought back, but he became a permanent addition to the main cast.
Commander Bumi only shows at the very end of the finale, screaming like a maniac (just like his namesake). The fandom loves him. His expanded role in Season 2 only deepened that love.
Tarrlok, whose Murder-Suicide of him and his brother Amon/Noatak has cemented him in the fandom of an Alas, Poor Villain. Many fans wish that he had lived and would have been able to undergo a redemption arc since it was obvious that he wanted to rise above the ghost of his father in his life.
This has even hit a background character with no name who is long dead, specifically an overweight Airbender Avatar with a marked resemblance to a Laughing Buddha seen in the Spirit World. He is, of course, the most powerful Avatar to ever live.
Angel from Lilo & Stitch: The Series, an intended one-shot who reappeared later due to popularity. Not to mention Sparky as well as some other experiments. Disney eventually acknowledged this and started Shipping Stitch with Angel in Asia.
Littlest Pet Shop (2012) gives us Sugar Sprinkles, the stoner-esque cat who lives in the Sweet Delights truck. After appearing in a single episode in season one and getting as much fan attention as the main pets, she came back for a single episode of season two, as well as another episode appearance in Season 3.
Looney Tunes has a fairly big history of this. Their first big star after Buddy and Beans The Cat was a minor character in a series of shorts about a group of grade school animals, but his role as the stuttering, somewhat unlucky Straight Man role proved popular with fans and Porky Pig soon became their premiere character. After a few years, a cartoon featured Porky going on a duck hunting trip, running into all sorts of wacky mishaps, one of which was an off kilter duck that only appeared in two scenes (and the credits), put his loony, unconventional antics proved so popular that soon that "Daffy" Duck got his own cartoons and became their next star, until the process repeated itself yet again with their now premiere character, Bugs Bunny.
One of the strangest examples: the nameless drunk cat with the red nose, big lips, saggy jowls, dickie shirt with bow tie and bandage on his tail. He only appeared in the Bob Clampett-directed "Kitty Kornered" as a pet of Porky Pig who teamed up with Sylvester and two smaller cats, but he proved popular enough to appear in many of the spin off series of the 1980's and 90's including Tiny Toon Adventures and The Sylvester and Tweety Mysteries.
Marvin the Martian also fits this only having a handful of short in the Looney Toons canon but ended up becoming the primary antagonist in Duck Dodgers and getting his own music video
Another example would be Taz the Tasmanian Devil. Originally he was only going to appear in one cartoon but he was saved by fans requesting he make more appearances; he made 4 additional cartoons and a major role as a villain in the Christmas special, proving popular enough to get his own TV show. He is now part of the main character rosters and one of the most recognizable characters in the series.
To a far lesser extent, Penelope Pussycat, originally the unlucky nameless cat who would get chased by Pepe Le Pew in his shorts has been gaining her fair share of popularity these days as well.
Speaking of Taz-Mania, it had a few as well, including The Platypus Brothers, Bull Gator and Axel, Hugh and Digeri Dingo.
If you ask a fan of The Magic Adventures Of Mumfie what their favorite character is, they will probably say "Scarecrow". He's such a beloved character that people who've seen the show as children refer to other people as "what-iffers".
In Metalocalypse, a show about the five members of a death metal band, who emerged as the most popular character? Their manager, Charles Foster Ofdensen. Fending off an assassin and making said assassin stab himself helped.
In Miraculous Ladybug, Nathanaël is just one of Marinette and Adrien's classmates who doesn't get a special amount of screentime or focus. However, he's the only one of their classmates who's a cute, shy, artistic boy with a crush on Marinette, something that the fandom has embraced enough to make him one of the show's most adored characters.
Despite the frequently horrifying content, the Peadofinder General in Monkey Dust is particularly popular. Something about the big hat and green skin...
Everest, spawn off a huge fan base before her debut episode even aired! She only appears in some but not all episodes, which places her in Ensemble Dark Horse territory.
Chickaletta surprisingly has a big fan base too.
Tracker was announced over a year before his debut, but at first, the fans didn't talk about him very much. After his debut in "Tracker Joins the Pups", however, he became very popular and was gladly welcomed into the fandom.
Brenda from Perfect Hair Forever has more fan art on at least Deviantart than anyone else in the show, and she seems to be relatively popular by its standards for her role as the Ms. Fanservice parody. Her usual outfit including a skirt that ends above the waist probably helped.
With the fanbase the size as it is, a case can be made for most (if not all) secondary characters, with the Fireside Girls (Gretchen in particular, since some fans pair her with Ferb) being the most prominent examples.
Eliza Beckham Fletcher, it is unlikely she will show up again yet she already got a sizable fanbase following less than 2 days after the one episode in which she does aired.
Vanessa Doofenshmirtz also has a huge fanbase, the reason why she has become a regular character.
As for background characters, the female backup dancer with the brown hair in the "S.I.M.P." song and "A Real Boy" has gotten a lot of attention. To the point where she got a speaking role and a Fanservice Pack in the Halloween special "Night of the Living Pharmacists". Apparently her name is Dana.
Planty the Potted Plant has accumulated a large fanbase too, including members of the staff who tried getting him more screentime. Unfortunately his scenes were mostly cut.
Similar to Vanessa, Norm was popular enough that his concept (mindless machine who spouts off trivial niceties) was rehashed into an intelligent robot to have him continue showing up.
Stacy. In the early episodes, she appeared only infrequently, if at all (she was originally just an unheard voice on the other end of Candace's cell phone conversations). She become very popular and as the series progressed, she appeared much more often, and has actually played a role in the plot of some episodes. Fan are very angry when she not appear in the finale.
Smokey from The PJs. Who ever thought that an animated series would feature a crack addict (and a lovable one at that)?
While most of the pups in the show were intended to be one-shot characters, some have gained some big fan bases. For instance, the puppy known as Pepper became a mega fan favorite (most likely because of her tough girl personality) so much that she was brought back in a Season 3 episode working with the Super Secret Pup Club.
Lucky's father, Slik, has an impressive fan base, mainly due to his striking resemblance to Tramp from Lady and the Tramp.
Cupcake started out as a dark horse. Everyone (both the fans and the show's creators) found her to be so cute that she quickly became a permanent recurring character starting in Season 2, except with a more down-to-earth personality rather than an innocent cute kid.
Lucky's owner, Dot, introduced in the Season 1 finale, has become likable enough that she would recur more often in Season 3.
The Rowdyruff Boys. They were flat, somewhat uncreative male counterparts to the three main characters. What's more, they were defeated by getting kissed. Mere days after the episode first aired, the Internet was swamped with dozens of fanfics and fanart pieces devoted to them. Entire websites - exhaustive websites, full of art, episode transcripts and bustling forums - were created in their honor. The fans were almost unanimous in the desire to see the boys come back. After four years of demand, they finally returned in season five - given hints of personalities and very strange new hairdos.
Bunny only appeared in one episode, yet is a popular character.
Many side and background characters are popular with fans, like Debbie Kang, Heidi Wienerman, Juggo, etc. Heck, even a nameless background character (dubbed "NG" for "Nameless Girl") is adored by some!
Cornchip Girl, Butch, and Lawson also have sizeable fanbases.
Miss Grotke, for being a MoeHippie Teacher with hints of Sensei-chan here and there (and was pretty popular among the show's older male fanbase)
The "Bad Kids" consisting of Conrad Mundy, Greg Skeens, Sue Bob Murphy, Kurst the Worst, and Lazy Kid are quite popular despite only having large roles in a few episodes. It isn't too hard to find fanfics with these guys in it.
Misery is far and away the most popular character on the cheerfully-gothic Canadian cartoon Ruby Gloom. Her only conceivable rival is the title character, at which point it's kind of hard to draw a comparison.
The Scotsman from Samurai Jack: a big, loud, barrel-chested walking stereotype with a huge Scottish claymore and a machine-gun for a leg who picks a fight with Jack for no real reason at all. Usually when a baddie does this he's curb-stomped by Jack in one blow, but fans were shocked and delighted when The Scotsman instead evenly matches Jack, fights him for a straight day, and the battle ends in a tie when they are both too exhausted to continue. The guy went on to return in a later episode, then gets A Day In The Lime Light episode where he saves Jack's life repeatedly over the course of a two-part episode.
Demongo got a pretty big fan base as well (despite only appearing in one episode), enough to even get him a profile on Cartoon Network's site at one point in their old shows archive. He also makes an appearance on the game FusionFall
X9. His episode was an excellent noir episode, and he had an excellent 1920's mobster theme. Shame he was forced to go against Jack... which only meant one thing.
Plenty of Scooby's examples of Monster of the Week have gone on to become pretty popular. Even to the point of being brought back for credit gags and video games. Some of the most notable ones being the Creeper, the Ghost Clown and the Tar Monster.
While most fans are not particularly fond of Scrappy, the series finale episode of the "Scooby & Scrappy" series, "The Ransom of Scooby Chief", introduced us to two old friends of Scrappy's: Duke and Annie. For one shot characters, these two pups gained a pretty impressive cult following with several fans, even spawning off some fan arts. One reason could be due to the great chemistry they had with Scrappy in the episode.
A lot of tertiary characters have become incredibly popular with fanfiction writers: Craig, Tweek, Pip, Gregory, Christophe (aka "The Mole"), the Goth Kids. Perhaps the most baffling is Damien, the son of Satan, who has spoken in exactly one episode and appeared in the background in a few others. Searching for him on fanfiction.net produces five pages of results, or over a hundred stories.
Damien wouldn't be nearly as popular if slash fans hadn't noticed his interaction with Pip, another example of this trope, in the one episode he was in. If you look closely at those fanfiction.net pages, you'll see that nearly all of them are Damien/Pip fanfics. Which is even more baffling, considering that Damien set Pip on fire to earn the respect of the other boys.
The popularity of the Goth Kids might be because fangirls like to use them as an outlet for their own angst—which is ironic, because that's the kind of attitude the Goth kids are supposed to be a parody of, with them doing it just to stand out and look down on others. Especially awkward since only one of them has a canonical name, though on Fanfiction.net at least "Dylan" for the red-tinged Goth and "Ethan" for the tall one seem to be fanon.
For a guy who is almost certainly gay and has only appeared in one episode and whose only relationship appears to be with Butters (meaning there shouldn't be much for fanfic writers to actually do with him) Bradley from "Cartman Sucks" has quite a few stories about him getting with Butters.
The diabetic Scott Malkinson has also gained a decent following, possibly attributing to his more frequent appearances in recent seasons.
Cad Bane earns this role as a badass villain with a western motif. Captain Rex has his fans too.
A lot of the other clone troopers are also pretty popular, partially because their episodes have more unpredictability in regards to their fates, giving them distinct personalities and acknowledging certain moral issues the Jedi leading them (and for fans that weren't very fond of Karen Traviss'snovels, doing so without flanderizing the Jedi Order's bad points). Notable examples include the boys of the 501st Legion, Waxer and Boil, clone commando Gregor, and Domino Squad - Hevy, Cutup, Droidbait, Echo and Fives; the latter two appearing in later episodes as part of the 501st Legion and becoming ARC troopers after surviving their debut episode, "Rookies".
As does Admiral Trench, the chessmasterspider who turned up, much to the surprise of Admiral Yularen who thought he was dead, made Anakin run behind the moon for cover, then bombed the planet to draw them out and deduced who was flying the retaliating stealth ship. When he died the first time it was at the hands of a special task force just for him and led by Jedi. Several fans lamented his apparent death at the end of his first appearance, enough for the writers to bring him back as a cyborg in "The Lost Missions".
Embo, the bounty hunter. After appearing in only one episode. This video demonstrates why. This led to him appearing in more episodes later on.
Savage Opress has proven to be quite popular, partly due to his resemblance to fellow Ensemble Darkhorse (and his brother) Darth Maul.
Hondo Ohnaka. He was supposed to be a one-shot villain, but was brought back twice in season two and then appeared in three arcs in season five. All in a row. You want to know why? THIS IS WHY.
Ruby and Sapphire, mainly because they just look so darn cute together and are a confirmed Gem pairing that explicitly love each other. They don't appear too often, but when they do fans are definitely happy to see them.
Sadie's consistently the most liked of the Beach City side cast. The vast majority of the side characters have their fans, but her snarky, upbeat, Only Sane Man role made her stand out. She'd later prove to be a Badass Normal as the first human to take out a Gem monster by herself, and have a endearingly shy personality when she's nervous.
Blue and Yellow Diamond's Pearls. They've only appeared on screen for about a minute, combined, and one of them doesn't even talk, but they amassed loads of fanart soon after their debuts.
Mystery Girl/S from the episode "Last One Out of Beach City". She's a cool, attractive biker girl with the punk look going on with pink hair, who quickly grabs Pearl's attention when her, Amethyst, and Steven go to a rock concert. Her fantastic character design, sheer mysteriousness as we don't hear her speak, and the fact that she's the first human to openly show attraction to a female-presenting Gem quickly made her a popular ship for Pearl. She only gives her inicial as "S" on her phone number, and the credits just call her Mystery Girl. Some fans have taken to calling her Shina from a nickname by one of the storyboard artists.
Initially only appearing once in a series with Quarter Hour Short episodes, Ultraprison's Mistress became surprisingly popular among the Superjail! fandom during its first season. She managed to return at the end of the following season, played a role in the third season premiere, and fourth season premiere involves Ultraprison yet again.
Charise also receives this treatment, due to her relationship with Jared and being seen as cuter and innocent compared to Mistress and Alice.
Tashy 497, a lovable squishy space squid who showed up in a single episode with the same name. Apparently, he was so adorable the fandom kept him, and now he has his own fanart, almost as much as anyone else, including the main cast. He often will make a cameo in other artworks, but never in series, because it ended and he died before the episode was over. 20 minutes screentime.
Xeexi. 2 minutes screen time.
Barb. She walks into their house on random, and has reached a supreme stalker status, even though she was only ever used to move the story along. Her stalker status came from her various intimate, and arguably sexual, encounters with Mr.Lunis. She's often depicted in fanworks as the creepy neighbor. So this is the one time where the fans keep a character IN character. 5-10 minutes screentime.
Mirabel. Mentioned because....there really is no reason why. She's just commonly mentioned. Went to the prom with Lance, then he left so she spent that time with Jason. Her popularity may have stemmed from people settling that Ilana was free again because Jason and Mirabel hit it off, even though that was never confirmed. minute and a half of screentime.
Kristin, the goth girl introduced in Ballad of the Scary Mary and asked out by Lance is Under the Three Moons. Because her potential was never completely reached, and because of her cool attitude, she became someone the fans noticed. Also rocketed in fandom for the same reason Jason is: She ruined the Lance/Ilana ship. Depicted in fanworks as a snarky, kickbutt, female Lance. Also popular because of an episode of Dexter's Laboratory, Eye Eye Eye's, where the little girl depicted looks like a younger version of Kristin. Now she's known as the Eye Eye Eye's girl. About 2 minutes of screentime.
The Eye Eye Eye's episode of Dexter actually made another character popular; Brandon. "Kristin" ran off with a brown haired boy with shining eyes, and he looks like a younger version of Brandon. And so the ship "Krandon" was born. His popularity also comes from his jerkish behavior towards Kimmy as his Ex. 45 seconds screentime. Both Titan and Dexter were created by Genndy Tartakovsky, justifying this assumption.
Todd has become popular due to a stellar performance by an avid RPer who took his little 30 second run and made an interesting character. Even though it was never said in the show, he says "fabu" alot in fanon, is bisexual, and looking for love.
Baron (arguably the biggest example of this on Titan's listings) from the episode Shadows of Youth has become popular enough to be shipped with Ilana, have his own fanfics, and many wanted him to show up again. In fanon, he grew up to be a sexy, yet confused, bastard who loves to torture Lance in his every day life, part of that being flirting with Ilana. He's actually blown up so much that him saying "I WAS SUPPOSED TO BE A PLOT DEVICE" has become a meme, and exploited for the fans to hold up their campaign. Part of the reason fans fight hard for this show is Baron, figuring out what happened to him, and the fact that you can't have such a jerk character who was depicted only as a child then never get the opportunity to show his later years and have no one wonder where he went, so his Darkhorsism is justified. 10 minutes screentime.
Mike Chan in "Roar of The White Dragon". Often depicted as the self centered, jerky, somehow sexy, Asian. Paired with Lance at times, even though Lance already outwardly expressed that he was in love with Mike's Car, but hated Mike. Might be popular because it was alluded that he would come back, but the show ended before he could. 12 minutes screentime.
His girlfriend, dubbed June by the aforementioned RP'ers (There is an official RP that does show cases on CN's facebook page with a well written out script, that is why they are so acknowledged and important in this fandom)even though she never said anything. And that is what she is known for. Not saying anything. 5 seconds screen time.
Meat. Popular because he was voiced by Bill Fagerbakke (Patrick Star). His wacky antics and nonsensical dialogue made him popular, and is often depicted doing something silly and out of place for this show. Also depicted as Octus' best friend even though he only had about a minute of screentime.
The other cheerleaders. Monica (the black one) is known for only talking about how hot Lance is, Amber is known as Kimmy's closest BFF who's not all that bright (likely brought about on her from the fandom because she's blond) and obsessed with her weight. Justified due to her freaking out and thinking an airway would break because she had eaten a single fry at lunch, and Tiffany, who is best known for not being known. At one point, people only noticed Amber and Monica, but when people started scratching their heads to remember the other one, when she was remembered, they remembered hard. Various amounts of screentime, but no more than 10 minutes.
Argent. Despite only appearing for a short amount in two episodes, and subsequently being a very little developed character, there's a large amount of fanart for her, and several fanart groups on DeviantArt.
There's also Kid Flash, who also only appeared in two episodes. Probably helps that he and Jinx became an Official Couple.
Mona Lisa, a girl mutant from Teenage Mutant Ninja Turtles (1987), appeared in just one episode in Season 4 as a potential love interest for Raphael. She remains one of the more popular side-characters.
This show's version of Karai has developed a following. Her loyalty to the Shredder/Foot is downplayed, her snarks are hilarious, and her Ship Tease with Leo helps (or doesn't).
Xever/Fishface is slightly more popular than his partner Chris Bradford/Dogpound, due in part to having a more visually appealing, unique fighting style. It probably helps that the audience learns far more about him than they've learned about his partner.
Spike and Murakami, thanks to being a funny background critter and a Cool Old Guy respectively..
Despite starting out as The Scrappy (mostly due to shipping), Casey has since become a fan favorite.
Starting out as divided when people found out about the character Slash is now very well liked due to being portrayed as a cunning, genuine threat who provides interesting tension with Raphael.
Ice Cream Kitty seems to have developed a following, most likely thanks to the mutant kitty being genuinely adorable, especially whenever Mikey's interacting with her.
Tiger Claw has some of this due to being a Badass bounty hunter who puts up a brutal fight with the turtles.
Kraang Sub-Prime, for finally giving this show a Kraang in the style of Krang from the 80s cartoon.
Bebop and Rocksteady, two old fan favorites from the old show, now remade into more dangerous villains with richer personalities than ever before.
Noah is incredibly popular with the fandom despite being one of the most minor contestants in the series (he was booted off third in the first season, wasn't even a contestant in the second, and got kicked out half-way in the third). His status is lampshaded by Chris, who introduces him and Cody as "returning favorites" in the first episode of World Tour. He finally got a major romantic plotline in the spinoff series The Ridonculous Race.
While he was reasonably popular beforehand, Tyler's presence in World Tour cemented him with almost similar levels of popularity to some of the other returning We Hardly Knew Ye types in that season.
Dawn's design was popular before Revenge of the Island even aired, and she's easily the most beloved second generation cast member (only rivaled by Brick), despite having very little screentime.
Brick is fairly well-liked by the fanbase, despite only competing for half a season.
Many thought Amy and Samey's subplot was far too short lived and wanted to see more of them.
Ella has had a similar reception to Dawn, especially after "Mo Monkey Mo Problems".
Beardo, of all characters, has a small but loyal fanbase, surely do to his being highly entertaining, and having a surprising amount of Hidden Depths for one episode of screentime.
"Billy" the intern, a blonde-haired character who appears frequently in Aftermath episodes, speaks only once, and doesn't even have a confirmed name.
"Emily," the flight attendant from the fourth aftermath of Action is quite popular.
Scruffy, Duncan's pet tarantula, also has this going for him.
The alien Cody from the Nevada episode, he even gained a nickname: Evil Alien Cody Clone, EACC for short.
Gwen's goth friends, Marilyn, Pixie Corpse, and Reaper, show up an awful lot in fan work.
Iron Fist is notable for being one of the few main characters to not be The Scrappy: he is less a jerk to Spider-Man than the others, he has an actual personality, and his origin actually is mentioned and shown.
Doctor Octopus; this version is outright creepy, has a developed and credible Jerkass Woobie backstory, is effective in a fight, has a good design and voice (even more impressive considering he is played by Tom Kenny!) and possessed an actual story arc that has him evolving. He seems to be the only dark and mature element so far that made it in the show.
Taskmaster and Beetle, being both portrayed as Bad Ass, menacing and truly competent, got quite good reception amongst the fans;
And so far, episodes involving other key heroes such as The Incredible Hulk, The Mighty Thor or Iron Man got good reaction as well due to being faithful to their most well-known incarnation, especially Iron Man.
Also, for those not mad about the change Harry Osborn is pretty popular since the idea of him being Venom is a different and interesting twist. The fact he also is pretty much possibly the nicest version of Harry Osborn in all animated adaptations may help as well.
Even those who dislike the show tend to admit the janitor played by Stan Lee is an enjoyable part in the show.
Deadpool was a welcome character. Since fourth wall-breaking and shameless comedy have been major parts of his story and character for years, he was received very well. It helps that the episode was written by Joe freaking Kelly. One common praise was the simple fact that he could actually pull off the style of humor present in this show, which many fans hate, feeling that it's out-of-place coming from Spider-Man.
Pizza Steve from Uncle Grandpa, though given his personality is it any surprise?
The Venture Bros. has so many ensemble darkhorses that its hard to qualify. Minor characters re-appear as medium character, part of a team or full blown important characters.
21 and 24 (but especially 21 now) are the best example. They started off as nameless henchmen (numberless, too in the first episodes) to have more air time than the Monarch himself. In the latest season, 21, after he Took a Level in Badass, could arguably be called the leader of the Monarchs. The other henchmen refer to him a General 21. He is even putting the moves on Dr. Misses the Monarch, whom he has always crushed on.
Dr. Girlfriend started off as a minor character who eventually became a primary character and may have more screentime than The Monarch
Sergent Hatred and Monstroso ares first introduced as just a name along with other generically named villains before becoming Dr. Venture's Body Guard and primary antagonists respectively.
Baron Von Underbite, Scott Hall and Radical Left were medium, minor and cameo characters respectively who become part of The Revenge Society who play a major part in the plot
Scott Hall was a basic yet competent minion who eventually became a powerful super villain who killed dozens of heroes and villains and then joined The Revenge Society.
Shore Leave and Sky Pilot were first shown as OSI agents briefly in a flashback alongside several other OSI agents, and then becoming minor characters for an episode who help Brock escape assassins. And then Shore Leave becomes a relatively important Sphinx agent alongside Brock and Sky Pilot turns out to have been an important mole all along.
Sphinx and a few of its members appeared in a brief flashback before the good guys revive Sphinx to hunt down bad guys and a few former Sphinx operatives get an entire episode dedicated to them.
The OSI and The Guild of Calamitous Intent were originally unknown to everyone in the cast and were super secretive organizations only heard of in whispers.
The Sea Captain was a one episode villain that is now a reoccurring minor character.
The Investors, Cardholder and Doe, and Molotov Cocktease all start off as memorable characters in their own right but become absolutely vital characters that entire season/series plotlines revolve around.
The third Wallace & Gromit short, A Close Shave introduced a small gluttonous lamb as supporting character who gained quite a fan base. After helping chain off a great deal of toys and merchandise for the series, Shaun the Sheep was later granted his own TV series and set to have his own feature film.
Shaun himself spawned his own Ensemble Darkhorse in the form of even smaller cuter lamb, Timmy, who again, earned his own cartoon for pre schoolers, Timmy Time.
Odlaw from the Where's Waldo? cartoon series seems more well-liked than Waldo himself. To a lesser extent, Woof could qualify as this.
The Heffalumps and Woozles from The Many Adventures of Winnie the Pooh. Canonically, they only appeared in one 3-minute musical segment and were never mentioned again within the film. In addition, they have rarely ever appeared in the Disney Crossovers and for the ridiculously few times that they were, they were just a pass-by halloween segment or mooks at best! Despite this, they have gained love (and/or terror) from fans of every shapes and sizes due to the segment's love of anything that comes out of a LSD hookah.
Probably the most overlooked character in the whole franchise is Gopher, a character who is very humorous and likable. What makes him so unique is the fact that he was made specifically for the Disney franchise and was not from the stories by A.A. Milne (he even jokes about it by saying, "I'm not in the book.").
Diaspro and Chimera count too, at least in their first appearance when they really looked threatening.
The Wizards of the Black Circle, due to being the most competent villains in the series, and the only ones to permanently kill one of the Heroes. The fan base was pissed at how easily they were defeated.
Daphne is beloved for being a Cool Big Sis, having a unique backstory, and having a really interesting design, though many were upset when she was revived.
Woody Woodpecker originally debuted as the villain of the Andy Panda cartoon short Knock Knock in an attempt to deliberately invoke this trope-Walter Lantz and his cartoon studio were hitting hard times after the series Oswald the Lucky Rabbit ran out of gas, and Andy Panda wasn't the new hit star they needed badly to rejuvenate the ailing studio. Unsurprisingly, Woody became an instant hit upon debut and went on to immediately star in his own smash hit series of short subjects.
Beast. Eric Lewald states that the writers considered him a guest character in Season 1, which is why he was largely sidelined. Fans and writers alike took to the character, leading to his increased prominence for the remainder of the series.
Morph was in the first episodes entirely for the purpose of being killed off so the writers could show how serious the series would be. However, his unexpected popularity led to him being revived in the second season.]
Nightcrawler's episode was so well-received they wrote a second one just to bring him back and the writers seriously considered having him join the X-Men after "Bloodlines."
Which would have been quite appropriate, seeing as how Nightcrawler was part of the original New X-Men lineup from the 70s — alongside Wolverine and Colossus.
An example of a team of Ensemble Darkhorses are the Brotherhood of Mutants. Particularly Quicksilver and Toad. While they as a whole get it for their status as Ineffectual Sympathetic Villain, the Fan Dumb tend to be very touchy on this. While undoubtedly entertaining and easily popular, they have a habit of being latched onto by the crazier fans. Especially Avalanche and Quicksilver.
Canon ImmigrantX-23 was so popular that she was brought into the comics after only two real appearances and a cameo in the finale, with a million fanfics dealing with her life.
The show has a notable exception among the franchise: Wolverineisn't the most popular character on the heroes side. There actually isn't just one in particular. The show was so well written than almost anyone could function as the Ensemble Darkhorse, and if you look at the fandom that's what happened, but special mention on the good guys side should go to Nightcrawler. This is about as close and any adaptation got to his original playful, charming, character from the comics.
Nightcrawler is easily the most popular male of the main cast and the most popular female character in the show is Rogue.
Yuck from Yin Yang Yo! is the evil counterpart of Yin and Yang whose both a Manipulative Bastard and a Jerk Ass. He only appears in six episodes total throughout the whole series yet that doesn't stop him from having a much larger fanbase than the main characters. Though other villains such as Carl and Smoke are fairly well liked, he is arguably the most popular. His Foe Yay with Yin in "Falling Yin Love" and Heel–Face Revolving Door in "Upstanding Yuck" only add on to this.
While people are divided about the characterization of most of the cast, particularly Superman and The Joker, there's one character nobody dislikes: Captain Marvel. He's the most fun character in the cast and seems to be be as popular with the fans—and even non-fans—as AQUAMAN was.
The runaways who were experimented on by the Reach (Eduardo Dorado, Jr., Tye Longshadow, Asami Koizumi, and Virgil Hawkins) achieved this after "Runaways". Virgil already had a built-in fanbase, being one of the most hoped-for characters since the show premiered. But Eduardo, Tye, and Asami, derived from Superfriends, have already garnered plenty of fans on their own. That they're all True Companions already helps.
"Coffee Pot Guy", the unmasked manta trooper who offers Tigress a fresh pot of coffee in "Complications".
Nuka and Vitani have relatively minor roles in The Lion King sequel, yet have become the favorite of many fans likely due to their unique designs and obscure backstories (or rather a lack of).
Hunter from Father of the Pride. Despite being a short-lived show, he's one of the most well-liked characters in it.
Fans of The 7D have proclaimed that the Glooms are the "real" stars of the show.
The Tick has the outrageously funny and silly The Evil Midnight Bomber What Bombs at Midnight (Yeah, baby, yeah!). His hilarious patter and outrageous behavior made him a fan favorite. Just watch starting at 0:57.
Jasiri from The Lion Guard is this for being a good hyena, and having (arguably) the best song in the show.