A subtrope of Original Character. An OC Stand In is a canon Flat Character that gets little screentime or few (if any) lines, and therefore can be molded into whoever the fans want. Often, they may not even have canonical names. Thus an OC Stand In is like an OC, but the fact that these characters usually have at least some personality already means the author has some general direction to go in. Like all things in fanfiction, when handled badly the OC Stand In can delve straight into Mary Sue territory. Compare with the Possession Sue.
Sometimes these characters are just used to insult the heroes and make life hard for them, giving the character little point. Does Harry Potter really need another teacher who's out to get him? Hasn't Naruto dealt with enough crap already? In these cases, the character is simply used to replace the Canon obstacles for The Hero with smaller and (sometimes) more realistic ones.
An OC Stand In is sometimes used to save a main character from being bashed... The OC Stand In is then bashed instead, like how, for example, many fanfic writers make Sakura's mother responsible for her being a Fangirl and an Alpha Bitch and let her make an whole laundry list of crimes against Naruto and the village.
However, there are cases, even when the OC Stand In is "high school evil", that these characters turn out likeable. Or, at least, the Fan Fic does.
In video games, sometimes the canon protagonist is an OC Stand In, due to being a Featureless Protagonist, a Heroic Mime, and/or a highly customizable RPG player character.
A particularly successful interpretation of one of these is likely to ascend intoFanon when other fans want to play with that version.
Also known as an "Original Canon Character" or OCC.
Related to Memetic Bystander.
Kishimoto, oddly enough, said he likes her design most out of the female ninja.
This becomes even more embarrassing when you see the number of fanfics. She has much more fics than any other secondary characters with little to none screen time, like Choji, Shizune and Shino togheter, and more than other characters with full fleshed personalities, like Rock Lee or Kiba, and even Kabuto who is a major character. Her personality varies with the author, but you got to admit, it's quite impressive for a character who doesn't even have a full name.
Sakura's parents, particularly her mother, are common Fan Fic characters. Sakura's mother is heard off-camera in the anime, but she only seems to exist to prove that Sakura, unlike Naruto and Sasuke, had a normal childhood. In fan works, she is usually both a Straw Civilian and a total bitch to Naruto — someone the Naru/Saku shippers can use as a Freudian Excuse for Sakura, or just plain hate for no reason. Both of Sakura's parents finally appeared and received names in the movie Road to Ninja.
Other members of the Seven Swordsmen of the Mist show up often as oneshot villains. They're not even real characters to stand in for, so much as placeholders where characters presumably go.
Similarly, tailed beast hosts number 2 and 4-7, as two and four were only seen briefly while the others were all killed offscreen. The fact that the author made full designs of all of them (even for the three-tail host, when the three-tailed beast didn't presently have a host) for a cover as part of a Milestone Celebration is pretty much asking for this — though ironically at least one of them has been given a plot-relevant backstory post-mortem (the three-tails was the last Mizukage).
Yugito Nii (the 2-tails host) is an extremely popular OC Stand In, and is a frequent shippingtarget for Naruto. Fuu (the 7-tails) is also becoming popular in this way after her design was revealed.
Also, Hanabi Hyuuga, Hinata's sister, who only speaks in one scene and primarily only asks questions of her father. Fans tend to differ on whether she would become the heir or a branch house member (and how well she would accept either), and whether she loves Hinata or sees her as worthless.
Or, for that matter, sees her as a threat - the natural consequence of the prospect of one sister enslaving the other or else dying.
Naruto's mother also got this treatment - either as a placeholder (like the swordsmen), or a true example once Kushina was first seen, even though we only got to see her with her pregnancy-bloated belly and hear her say one line in her first appearance. One popular characterization is that she was a tomboy in her youth - which turned out to be dead accurate when her soul appeared to help Naruto gain control over the Kyuubi's chakra.
Dragon Ball Z: Vegeta's father gets this treatment. He's been dead for years and gets three scenes total over the course of the anime. He gets another in the first Broly movie, but if you count it as canon (which requires some shoehorning) it's a Moral Event Horizon. He shows only bare hints of personality. Yet he's also the subject of fan devotion that's surprisingly intense, if small in scale. (Vegeta's Missing Mom is also the subject of much speculation.)
On a related note, the Chick of Bardock's squad, Fasha/Seripa sometimes overlapped with the "Goku's Mom" role. She, and all of Bardock's teammates, are frequently OC stand-ins in their own rights.
Happens very often in Axis Powers Hetalia fandom with nations that have only appeared in profiles or on the author's blog, especially since fanfic writers can research these nations' histories and stereotypes to come up with some technical canon. Some of these common fan characterizations are:
Denmark as a Boisterous Bruiser and Norway as Tsundere for him because of their long history together; the former turned out to be pretty spot-on with canon, while the latter is still up in the air (though Norway did strangle Denmark with his tie in one strip).
Netherlands as similar to Denmark in personality with a pot-smoking habit; this was one of the fanon characterizations that ended up Jossed by canon when Netherlands was revealed to be a mainly blunt and stoic Perpetual Frowner, though the smoking habit was pretty much confirmed.
Hong Kong as an Emotionless Boy with a penchant for pranks and snarkiness; this was mostly confirmed by canon, although he doesn't seem to be quite as emotionless as fanon loved making him out to be.
Taiwan as a Yamato Nadeshiko with romantic feelings for Japan; the Yamato Nadeshiko part was Jossed with the Drama CD (and more recently, the fourth manga volume) showing her to be a PluckyGenki Girl, and she doesn't seem to act differently with Japan than she does with her other Asian siblings.
Romania was almost always depicted by fans as a female (probably because of the potential Les Yay between her and Hungary), only to have canon Joss that with the fourth manga volume establishing him as male.
And with the canon Romania appearing to have red eyes and Cute Little Fangs, it's becoming common to depict him as either a vampire or an ordinary human who pretends to be a vampire for the lulz. It also helps that Romania is the country Transylvania is located in.
India, like Romania, was almost always depicted as female in fanworks only to be established as male later on in canon. No one seems to really care, though, probably because of his awesome Bollywood entrance in Hetaween 2011.
Matt from Death Note is a good example of this. He gets little time on the page in the manga, and even less time in the anime, but he has a large fan base, and thus many people end up writing about him, usually just writing him as whatever they need, within limits, for the story.
Also Linda, who was mentioned in the manga a grand total of twice, seen only once, and has only one line. Considering that, she is a massiveEnsemble Darkhorse; if you type in "linda death note" on deviantART you get 526 results, and she has 34 fanfictions on fanfiction.net.
Yamamoto, Light's friend from school who later becomes Matsuda's partner at the NPA and has Ho Yay with both. He gets about as many scenes as Matt (though he's not nearly as insanely popular) and doesn't appear at all in the anime but he does show up regularly in fic.
There's also Misa's sister, who is mentioned in the manga and may or may not even exist.
The spin-off novel Another Note (which was not written by Ohba, but was approved by them) gives readers the character of "A", the first Wammy's House child who committed suicide out of pressure of being L. Unlike B/Beyond Birthday, we get nothing in the way of A's personality or backstory, leaving fans to have to fill in a lot of blanks when they utilize him in ther fanfic.
Though they go unnamed in the manga, Ranma 1/2 has Ranma's buddies Hiroshi and Daisuke, and for the girls there's Akane's friends Yuka and Sayuri. They mainly exist to be Those Two Guys. Rumiko Takahashi seems to love this trope, especially in Ranma. Herb's got Mint and Lime. Kima has Masara and Koruma, and is herself not much developed beyond her role as Saffron's bodyguard. The Anime also gave us Kaori Daikoku, who appeared just once in the anime and yet comes up rather frequently in fanfiction.
From Neon Genesis Evangelion we have Kyoko Soryu (we have no idea what she was like before theincident, or even what the woman looks like) and (to a lesser extent, due to having somewhat of a discernible canon characterization) Yui Ikari.
Also, Class Representative Hikari Horaki's sisters, Kodama and Nozomi; they never appear on-screen, and the only known facts about them are that they're older and younger (respectively) than Hikari and that the three of them have a Missing Mom like the rest of their class. To a lesser extent, Hikari herself.
Touji's little sister sometimes got used this way even before her first onscreen appearance in the second Rebuild movie. She became even more important in the third movie, finally being named onscreen (Sakura Suzuhara).
The unnamed girl from The Dissociation of Haruhi Suzumiya was often used as this in Haruhi Suzumiya fanfiction until the release of The Surprise of Haruhi Suzumiya, where she was given a name (Watahashi Yasumi) and a backstory which Jossed pretty much all interpretations of the character overnight. Less commonly, the unnamed boy with glasses whose life Kyon saves in The Rampage of Haruhi Suzumiya is used for authors who want to write about Kyon's sister's age group.
During the Fullmetal Alchemist movie, and around the end of the show, Ed gets sent to "our" world where everyone he knows has an Identical Stranger. Fans have a ball with the unseen ones such as Winry's, who is usually portrayed as the opposite of tomboyish counterpart and is either a Dark FicReplacement Goldfish or someone Ed learns to love, and Roy who is almost always portrayed as the Jerk Ass.
In Code Geass, Kallen's brother Naoto Kozuki sometimes get this treatment, largely because all we know about him is that he was the leader of Ohgi's group and died sometime before the series started.
In Yu-Gi-Oh!fandom, there was a brief period of time during which people knew about the Toei series, but had no fansub available. Cue Miho, who was a one-shot character in the manga and had a prominent role in the anime, becoming an OC Stand In, though her personality did often overlap with the one she had in the manga.
Yugi's parents are also frequently subject to this in fanfiction. In canon, Yugi's father never appears, and the most known about him is that he's a traveling salesman who's always away on work, while Yugi's mother appears only once in the manga and twice in the anime. Her second appearance is cut entirely from the dubbed version, however, hence she isn't really remembered. Fanon characterizations of them vary between abusive, deadbeat, Good Parents, or dead.
This is heavily subverted with the series Zatch Bell!, where unless you're Zatch, Tia, Zeon, Sherry, Brago, or another of the main characters, you'll never get any actual writing or stories based around you. Case in point, any of the many villian-a-week enemies of other mamodos Zatch faces. This extends to major recurring enemies as well.
Game Theory gets bonus points for it's use of the giant cat from Episode 3 (yes, the trope image). In the fic, she's named Vesta and after a series of events she becomes Nanoha's familiar. After being shrunken down, of course.
We knew Tokyo Mew Mew A La Mode was coming long before we saw it, so a lot of fanfic authors pounced on the new team member who was, for the nonce, a blank slate. Unfortunately, this meant the name "Berii" (and several spelling variants) appeared on a lot of Mary Sues in hopes of giving them legitimacy. More unfortunately, the truth of Berry Sue was even worse than (fan)fiction.
There are a few other Cyborg in Gunslinger Girl besides the main girls however they're not given significant screentime.
ThisX-Men fic is one of the rare Possession Sue cases. It takes a very obscure character (Magneto's canonically-dead wife Magda) and turns her from a meek woman whose only real moment of characterization was fleeing her husband after he accidentally killed a bunch of people into a de-aged (since, as a Holocaust survivor, she should be fairly old, even with Comic Book Time) genius roboticist mutant with a power that's supposedly completely unique and desirable enough that people would kidnap her for it (despite there already being a canon character with the same power whom no one makes much of a fuss over).
Unused or non-canon characters from The Lion King series are especially apparent in fanon works. The most common are Mheetu (Nala's brother), Kopa/Fluffy (Simba's son), Tesma (a meerkat friend of Simba), Naanda (Sarabi's sister), and Bhati (a bat-earred fox friend of Simba). Given that most of them don't have much of a personality this leads fan-artists to create their own versions.
Sarafina, Nala's mother, and Vitani from the sequel are popular choices too.
Uhura's Orion roommate Gaila from Star Trek (2009) had only a couple of minutes of screentime and a Wordof God backstory as having escaped the Orion slave trade on an "underground railroad" of sorts. Fandom took her and turned her into a sex-positive, feminist symbol through which authors often explore cultural taboos regarding sex as well as culture shock and culture clashes.
Blaise Zabini gets special mention here since from his name appearing in book one until he got a physical description in the book six, half the fanfic writers assumed he was a girl, since "Blaise" is more commonly a girl's name in some places. (They've since taken to writing about Daphne Greengrass or Tracey Davis when they need a generic Slytherin girl.)
This even applies to translators. In the Dutch translation, names are frequently changed to sound more familiar to the readers. So a Bella Zabini was sorted in book one, and in book six 'she' magically became Benno Zabini.
Daphne Greengrass also gets a lot of attention when authors need a sympathetic Slytherin girl, as she's rarely if ever mentioned in the same negative light as Draco or Pansy in the books (and probably because Draco hooks up with her sister after his Heel-Face Turn). Typically she's said to come from a neutral wizarding family or otherwise isn't connected with the rest of her house, possessing the same traits that Slytherins are known for but without invoking Ambition Is Evil.
At the time of this writing, Marlene McKinnon has 800+ stories on Fanfiction.net. Who is Marlene McKinnon? An Order member who died in the First War, along with her family; Lily mentions having cried all night when she heard about her death. Fans have (somewhat reasonably) extrapolated that the two may have been friends, and (less reasonably) concluded and that Marlene was Sirius' girlfriend/wife/etc., which is where most of her popularity comes from.
Chris Columbus giving his daughter Eleanor The Cameo by casting her as Susan Bones in the first two movies might almost count as an "official" version of this. After all, Susan was a background role with no lines, so what better place would there be for the American director to slip his daughter in without effecting the all-British cast requirement? Well, true to form, Susan ended up getting developed into more than just a throwaway name in the books written after the first two movies came out. The latter films handled this by simply not including her plotline, which they likely would have done anyway considering the length of the latter books.
Back before Ginny had much of a canonically defined personality, she was a frequent target of this.
By the same token we have the spouses of the characters who Hooked Up Afterwards. Draco's wife Astoria/Asteria Greengrass is usually either a sympathetic Slytherin or a victim of Die for Our Ship. Percy's wife, Audrey, is either a sweet girl or a rebellious firebrand in contrast to her husband; she may also be a Muggle. Luna's husband, Rolf Scamander, is implied by Word of God to be an adventurous naturalist.
Hermione's Invisible Parents. The only thing known for sure about them is that they're Muggle dentists. (Although at least one fanfic has them as doctors specializing in oral surgery and Hermione just explaining that as 'dentists' to wizards.) As long as that requirement is fulfilled, fanfic writers can fill in whatever they want. Even their first names are up for grabs.
Aeneas has a very minor role in The Iliad, but Virgil made him the protagonist of The Aeneid, partly because he was such a blank slate and partly because he was one of the few Trojans to survive the sack of Troy. This makes this trope, you guessed it...
In Peter Pan, Jane appeared in only one scene and was basically a Generation Xerox of her mother, Wendy. The Disney sequel Return to Neverland turned her into a highly pragmatic, disillusioned girl who grew up too fast as a result of living through the bombing in London. Margaret, the next descendant in the Wendy line (named, but never shown), gets this treatment in some fanfics.
She also made a two episode appearance in Peter Pan & the Pirates when Peter brings her to Neverland as a new Lost Boy only to find out about her being Wendy's daughter from the future.
The past competitors/victors in Catching Fire are often this, since few of them are given backstories. Fics about how they won their respective games are also common.
Also, many of the other tributes from Katniss's games have be used this way. Glimmer, Marvel, Thresh, the girl from 4, the boy from 3 who dug up the mines, and the girl from 8 whom the careers killed the first night, are all distinct enough that they aren't completely anonymous, but have almost no defined characteristics or personality.
The Starlings from The 39 Clues served as this in many fanfics, having been Put on a Bus early on in the first book. That book revealed little more than the fact that they were rich triplets and not very nice, and Word of God later claimed that they were part of the Ekaterina branch of Cahill family, but other than that, fans were free to fit them with whatever characteristics they wanted. Then The Bus Came Back.
Terry Pratchett's Discworld has dozens and dozens of spear carriers, cameo roles, and placeholders. Named Assassins such as Alice Band, Miss Smith-Rhodes, Madame Deux-Epees, for instance, or the hero and adventurer Howondaland Smith. Characters like this are fertile turf for the fanfic writer. Howondaland Smith gets a tale of his own in The Black Sheep, which also features Miss Smith-Rhodes and Madame Deux-Epees in their fuller forms.
Live Action TV
There are LOST fanfics that takes the minor survivors, including extras, and gives them the full on Lost treatment, with flashbacks and their own sideplots.
Ronnie Gardocki from The Shield qualifies as a major example of a live action version, as fans of the character have largely grafted onto him the personality of good guy nerd/geek who fell in with the wrong crowd as far as personality goes. Ironically, David Rees Snell (the actor playing Ronnie) has joked that had the writers fleshed out Ronnie and explored the character's dark side in relations to him being just like Lem, Shane, and Vic as far as being a Jerk Jock, that he probably would have lost all of his fans.
iCarly: Rebecca Berkowitz, seen only once, in an extended rarely seen version of "iSaw Him First" and usually mentioned off-handedly. In any fic with her as a character, she's basically going to be an OC with a canon name, if they even bother to develop her past the implied Really Gets Around.
On Neighbours, Lisa Jeffries was a very minor character who shared a few scenes with Summer Hoyland; she appeared in about a dozen episodes over the course of two years. She has, however, inspired a substantial volume of fanfic, which has developed her in ways totally unrelated to her canonical characterization.
A juvenile character named Marissa Flores, who appeared in the Star Trek: The Next Generation episode "Disaster" (and absolutely nowhere else in Trek canon), spawned the rather infamous Marissa Picard fanfic series, written by a chap named Stephen Ratliff. And it's not limited to just Marissa; Ratliff also included a bunch of one-off kid characters from TNG as the title character's various minions and cohorts.
In the 30 Rock fandom, the Liz/Gretchen ship has a sizable following. Even if you're regular viewer of 30 Rock, you're probably wondering who Gretchen is. Well, she's a "brilliant plastics engineer/lesbian" whom Liz was set up on a date with as part of a Mistaken for Gay plot. Gretchen appeared in one episode back in the first season and has not been seen or mentioned since. But let's face it, it's not hard to see the appeal of pairing someone who looks like Stephanie March with someone who looks like Tina Fey. Plus, the episode contained a substantial amount of Les Yay, featured Frank and Pete becoming Shippers on Deck for Liz/Gretchen ("So you're saying I should just be a lesbian?", "I'm not saying it'll be easy. You get drunk first."), and ended with Gretchen deciding she couldn't be Just Friends with Liz because they were getting into a Stupid Sexy Friend situation.
On Glee the Warblers have little to no personality. They're very popular with fanfic writers.
Also, Puck's mother and sister. Since they don't even have names in canon, but he presumably lives with them, they play at least a minor part in most fics about his life outside of school and glee.
In The Vampire Diaries, it's canonically stated that Stefan and Damon had a sibling (usually a girl in fanfics) that was born from a different mother. Whether or not the writers of the Salvatore sister stories actually know this, however, is unknown.
Also Katherine is shown to have had a younger sister in her origin story that was killed by Klaus. Commonly she is not actually killed but rather turned by him. Katherine also had a daughter that could have been turned after giving birth to a child.
Vocaloid, naturally, since there is nothing canon about the singers, besides name, voice and appearance (and sometimes not even that).
One story managed to bring in one of the character's creator as a separate character.
The Little Red-Haired Girl from Peanuts is an unseen character, though she made a few tantalizing non-canon appearances in television specials), and we know nothing about her personality since Charlie Brown doesn't have the guts to talk to her (and, hence, really knows nothing about her himself. Any time she appears in a fanfic, she is this trope by necessity.
Peanuts is actually full of minor characters who get used as OC stand-ins. Of particular note are Franklin, Violet (usually Alpha Bitch, especially in stories that require Lucy to be slightly nicer and need a replacement villain), and Shermy (whose character trait in most fanfics is having no character traits at all, unless he's The Ace to Charlie Brown as he was in several early strips).
This trope is all over the place in Transformers. The franchise is filled with characters who only appear as toys with bio notes of varying character depth, and occasionally toys without bio notes. It has even happened in canon on a few occasions - the profile book More Than Meets The Eye required the authors to invent personalities for the Micromasters out of whole cloth, since those characters did not have individual bio notes, only bio notes for their team.
The same applies to BIONICLE and its Loads and Loads of Characters, some of whom only appeared in a scene or two and barely get to be described (the author claims he is very bad at coming up with visual traits, for example). The developers of the beloved Mata Nui On-Line Game said that LEGO barely gave them the most minor of side-characters to work with, and they had to develop their personalities and culture themselves. What resulted was a game teeming with characters more memorable and charming than many of the ones that the "official guys" made up. They easily became fan favorites.
Fans themselves also get chances to flesh out some of the more underused characters, via official writing or art contests. In the latter, they can even design the look of the characters themselves.
Pretty much the point of My Little Pony. Often ponies who don't appear in the show (or just appear in the background) get toys. 1980's ponies had a brief (and often bizarre) story on their backcard, wheras more modern ponies just have a sentence about them (such as "likes to write stories.")
Golden Sun has a lot of this. Sure, you have some basic personality for all members besides the mute protagonist, but the character development and interaction between the party is almost zero, every time the team talks is about the next mission or quest. That leaves a lot of open space for Fanon.
Just about any Pokémon fanfic that places characters from the games (Brendan, Lucas, Silver, Leaf, etc.) into the anime's continuity will make use of this principle to some extent, since most of said characters never appeared on the show outside of 3-second cameos, with Leaf never appearing at all. There's no set pattern for what personalities writers assign them, but there's usually a good chance that they will be used for Shipping somehow.
Likewise with game fanfics themselves. No one takes account to Dawn or May's in-game personalities, being that they're technically not their canon ones though. Most NPCs get this treatment to various effects. Rivals and protagonists are given whatever personality, being that you really have to look into the text of the games to see any bit of personality beyond "Hot-Blooded, determined Child Prodigy".
Touhou has this mainly in the form of the dialogueless midbossesnote except Te(w)i, who got plenty of canonical characterization later. The standouts are Daiyousei, Koakuma, and Momiji, who have managed to become fairly popular. Most of the time they remain Satellite Characters to the boss whose stage in which they appear.
Daiyousei and Koakuma don't even have any stated non-danmaku powers in a series where virtually everybody has a random superpower, so fans are required to make up a power for them. They didn't even have names originally, the ones used here invented entirely by the fandom (translated as "big/greater fairy" and "little devil", respectively).
Part of the reason for the massive doujin and fanfiction community that has arisen around the games is that, despite the Loads and Loads of Characters, only a small portion of the cast receive anything more than a brief character profile and some dialogue, and even fewer receive any form of backstory, leaving writers with enormous wriggle room. Even the two Universe Compendiums are written by Unreliable Narrators working with a lot of second- and third-hand information, and ZUN himself is notoriously unhelpful and even outright contradicts himself.
Rin Satsuki is also prone to this treatment, being a Dummied Out character from Embodiment of Scarlet Devil. There is literally no characterization of her at all. In fact, there isn't even any artwork of her except for a few unused sprites in the game's code. However, most agree that this image◊ is probably supposed to be her.
Despite just being someone's quick M.U.G.E.N creation, the character "Sendai Hakurei no Miko" (The Previous Hakurei Shrine Maiden) was picked up by artists and fully fleshed out as the Miko who existed so long ago everyone has forgotten her name. Depicted as a Boisterous Bruiser with Boobs of Steel, she fights without Reimu's spell card rules. As such, she ended up Covered In Scars. Also depicted as a Mama Bear, depending on the artist.
Doctor Cain usually gets this treatment in Mega Man X. Despite being a pretty important canon character in regards to the backstory, he barely gets a few lines in the games before vanishing. Most fanfic writers characterize him as a kooky old man and a father figure to X and Zero. Lots of other characters could also qualify, since the MMX games were very sparse on characterization until the later games.
Dr. Cain finally did receive some characterization in Day of Sigma, a tie-in OVA for Mega Man: Maverick Hunter X; the problem here lies in the fact that MHX was something of a reboot for the series, the SNES games in particular, and as such, the cast received tweaks to their characterization (some minor, some major) which may or may not be incompatible with the later titles (plans for sequels fell through when the game underperformed). That, and Cain being killed off in a missile strike on Abel City ordered by Sigma, despite original canon stating that he was alive at least until the time of X4.
The same can be said for the characters in Mega Man (Classic) (most non-playable Robot Masters especially), since most of them only have a small character profile to hint at their personalities.
The Guardian Spirit from the first Baten Kaitos game is supposed to be you, and is therefore a sort of Featureless Protagonist. Naturally, there have been several fanfics written about the this character. Unfortunately, too many of these fics end up making the Spirit a female Mary Sue with a crush on Kalas. The term "Spirit Sue" was created by the fanbase to describe this particular interpretation of the character.
Marno from the prequel doesn't get this to the same degree, since he has an actual role in the plot. However, he could still work for this trope, since his actual personality is never shown.
The Forerunners from Halo were practically an entire species (or group of specieses) of OC Stand-Ins until The Forerunner Saga fully fleshed them out, jossing a whole lot of fan theories in the process.
The Rookie in Halo 3: ODST is this to a lesser degree, having no lines beyond grunts, no visible face (he always has his helmet on), and no identification beyond the initials "JD"; the designers claim this was intentional, to better allow the player to integrate with the character. Spartan-B312 in Halo: Reach is this even moreso, with male and female voice sets, and armor customization that is persistent throughout both campaign and multiplayer; this was done for the same reasons as the Rookie.
The "Library Girl with a Pigtail" in Final Fantasy VIII, who crushes on Zell and seems to have his interest by the end of the game, yet who remains nameless and without any personality beyond "bookish and shy" in canon. Begs for Possession Sue treatment by any Zell fangirl.
Odd canon example, Dissidia: Final Fantasy, a crossover that featured characters from the first 12 Final Fantasy games, had to pretty much create personalities for the Warrior of Light and the Onion Knight from the wholecloth. It should also be noted that the latter could have possibly been avoided if the game used Luneth or one of the other three orphans from the DS remake instead of the OK kids from the NES version, but that wouldn't quite fit what the game was going for.
Mass Effect fanfiction has Oriana Miranda's twin sister and Kolyat, Thane's son, as a relatively popular pairing despite never meeting, or even coming close to meeting and having no connection to each other in the game besides a certain Commander intersecting with their lives. Kolyat is developed a bit better than Oriana, who never actually speaks or is interacted with until the third game.
Preitor Gavorn -a turian whom you only meet outside of Afterlife- is this to an extent, too. He was largely ignored by the fanbase for some time, until a video in Lair of the Shadow Broker showed him with a human male in what appears to be a bedroom. Since then, he has been given varying personalities, though most have some hints of Straight Gay in them.
Another popular target for an OC Stand in is Hanna Shepard, the Player Character's mother if the spacer background is chosen.
Sonic the Hedgehog has accrued several such characters, though since these often turn up in the comics, said personalities are often used. Nevertheless, Ray the Flying Squirrel, Mighty The Armadillo (though he's almost always put as the lost forth member of Team Chaotix), Bean the Dynamite, and Bark the Polar Bear (often paired together, for being introduced in the same game), often end up being OC stand-ins, as do Ashura The Hedgehog, Wechnia, Tiara Boobowski, Nazo, and Honey The Cat, though these five are very rare to see in fanfic, not that the first four show up often, original characters are far more common.
A few shopkeeps from The World Ends with You get this treatment, and HT Masuoka in particular is a favourite of slash writers given his suggestive dialogue. Eri is the minor character who gets the most mileage by far, however, since she has a confirmed pre-existing relationship with Shiki and can be believably integrated into the main cast.
Kaede from .Hack://GU has absolutely no real importance in the story other than acting like Zelkova's bodyguard, although until she joins your party in the post game you have no idea what her weapon even is. The worst part is she basically vanishes in volume 1 after her appearance, yet you can still do the marriage event with her in Volume 3 despite her having absolutely no development and virtually no game time.
Fire Emblem has the tactician from the seventh game. His default name is Mark, but you can change that and his gender. He never says anything (that we hear) and is only spoken to. There are a small wealth of fanfics about him/her, and almost each one gives him a separate personality and back story.note Most popularly coming from Bern.
New Mystery of the Emblem and Awakening have My Unit/Avatar, a customizable unit who actually appears in your army during gameplay. They do actually have lines, though, but their personalities (particularly that of the My Unit in New Mystery) are somewhat nondescript. Naturally, plenty of fanfics have been written around these two (more so Awakening's My Unit than New Mystery's, probably because of No Export for You).
The originally-intended but eventually scrapped protagonist of Psychonauts, Dartagan, appeared in the game for less than five seconds. There is a huge range of interpretations of him within various different fanfics, roleplaying accounts, and even Creepypasta.
The player characters from the Harvest Moon series, being Silent Protagonists who's only real characterization is "friendly and hard working." Real OCs only come into the picture when fic writers want an exotic-looking character (90% of the time, female).
In Katawa Shoujo, the students in Hisao's class besides him, Shizune, Misha and Hanako get this treatment, as a piece of official art shows their names, club affiliations and disabilities, but nothing about their personalities or backstories.
Sasasha, a Chocobo raiser from Final Fantasy Tactics A2 had very few info on them. We never met them, just heard of them. This allowed some fanfic authors like the one for The Tainted Grimoire to shape the character as they pleased.
Ao Oni: has had a few over the various versions of the game (due to characters' roles becoming bigger, smaller, changing or being removed altogether), but Mika is the most consistent example. In the most recent version (6.23), she receives very little characterization beyond her attraction to Takuro before dying and coming back.
Introducing characters from the "classic" Spyro the Dragon titles into The Legend of Spyro reboot is a popular thing. Fans figure if Spyro and Hunter exist what's stopping characters like Bianca, Elora, or Ember from too?
In a more traditional way, certain characters such as Flame are barely in the games but are expanded upon by fans. Flame is often a loveinterest for Ember.
For the Mega Man Legends series, the Bonnes' parents show up in fics at times despite not appearing in canon.
The Midnight Crew Intermission of Homestuck brought us The Felt, a 16-man mafia group, only two of whom (Snowman and Doc Scratch) got substantial characterization to be clearly defined as characters. The remaining fourteen ALL get this treatment, as only their powers and maybe a single moment of funny is supposed to encompass their characterization. The only exception is Clover, whose appearances manage to characterize him as a fusion between The Riddler and the Clock King, which is still a fairly open characterization.
Until Act 6 Intermission 5, two of the Felt only appeared posthumously, and all that was known about them previously were their names and faces (of course, they still got no characterization at all when they showed up). While their appearances in fanfiction increased once they made an appearance in the story, there still were examples of people using them for as OC Stand Ins before anything was known about them.
The Post-Scratch Ancestors were intentionally written like this so that fanfic writers could have access to a more 'serious' Alternian setting than the setting of the modern trolls. Other than their basic appearance, most important actions, some of their Shipping proclivities and a bit of the personality of one of them (Mindfang, a sexy pirate and rather a bad writer), we know very little about what they are like. This is lampshaded in comic where Hussie refers to Doc Scratch's exposition on them as fanfiction.
Quill-Weave in Prequel. As par the course in the source material, she's given the slightest hint of a personality and features in the very first fighters' guild quest. She's much more fully fleshed out in the comic, especially through the scenes from her POV.
The same goes for Sigrid, who in Oblivion is a survivor of Kvatch who sells potions. Since the comic takes place before the events of Oblivion, she's made the head of the Mage's Guild in Kvatch and her unusually high Merchantile stat is explained as her being especially beautiful and charming (as well as using some artificial enhancements like Enchantments and Alchemy, mechanics that are known to be Game Breakers in the series).
Phineas and Ferb: Until episodes such as "Isabella and the Temple of Sap", the Fireside Girls had close to no lines and no characterization, other than Gretchen, who even then only spoke once or twice.
Especially Gretchen, who in fanon is often paired with Ferb, apparently just because she's the first girl his age (other than Isabella) to get a name.
Django counts as well. He's been used a lot more by fans than in the canon series.
Ezekiel from Total Drama Island gets this fairly often; he got very little development because he was the first character to get voted off, an event that some viewers thought was unfair since he had completed the episode's challenge while other characters didn't. To a lesser extent Katie and Sadie also get this, since in canon aside from their co-dependency they have very little that separates them from the more developed and funnier Lindsay.
Gretchen on Invader Zim. In canon, she was strongly hinted to have a crush on Dib in one episode. The pairing is not widely popular (despite being the only romance in an otherwise No Hugging, No Kissing series), but even those who don't use her as a main character often elevate her to "Most Important Random Skoolkid" or mention her and Dib's "romance" in passing.
Another commonly used character is Invader Tenn; she only shows up in two scenes in one episode, and it was planned for her to return in the never developed series finale. Because of this, she is very often often used whenever a writer wants or needs to use an Irken outside the ones with more screen time (Zim, the Tallest, Tak, Skoodge). Of course, most of the stories she shows up in are based on (or are outright fan versions) of the aforementioned finale.
Zita is another victim to this. While a girl with her looks is common to see in the episodes, all we know about her is that she considers Dib to be crazy and, unlike other students, is respected by Ms. Bitters. Nevertheless, people throw even this minor canon out of the window and pair her up with Dib.
The Metalocalypse fandom has Klokateers. In the show, they're several legions of hooded, uniformed, fanatically loyal employees of the Dethklok company who rarely have lines and are referred to by number rather than name. Because of the extremely stringent entrance exam and its 50+% mortality rate, the recruits who survive to become Klokateers are extremely exceptional specimens, physically and intellectually. This makes them ripe for this trope. There are a few who have risen to prominence, mostly through slash fandom.
Daria fandom is pretty notorious for this, largely due to the fact that the show has scads of random recurring backgrounders who never do anything and yet have extremely distinctive character designs (a result of them all being based on real people who worked at MTV at the time). Because the fandom is relatively close-knit, the characterizations of many backgrounders have actually become pretty standardized. Burnout Girl is almost always involved with drugs, that blond from The Invitation is named Tori Jericho & was president of the Fashion Club before Sandi, etc.
Special mention goes to Princess Luna. When she finally appeared in Season 2, she left this trope behind by simultaneously confirming AND contradicting nearly every fan depiction of her.
Because of the hugePeriphery Demographic that follows this show, many of the background characters have Word Of Dante Style personalities that are almost universally followed. Even before the comic series more or less made many of these personalities canon, it was strange to see Octavia as anything other than a cultured, sophisticated character with a British accent or Vinyl Scratch as being a wild unpredictable party animal, and the duo would always be depicted as at least friendly rivals and at most lovers. Keep in mind these two characters had no lines and no interactions whatsoever on the show and never even appeared in the same episodes. They're only two examples: almost every background character ever has been given a back story and a characterization by the fans, and some are popular enough that it is considered Fandom Heresy to go against them.
The most prominent are Lyra, Bon Bon, Vinyl Scratch, Octavia, and Derpy Hooves, with Button Mash (a character who literally showed up for one second in one episode) getting a spin-off online animation which inspired a large number of fanfics about him as well. Soarin also frequently appears in this role, doubly so because he is a rare male character on the show.
It's common with the other series too. There are dozens, maybe even hundreds, of toy characters that either didn't pop up in the cartoon adaptations or didn't get much screen-time at all (many didn't even speak). Thus people make up whatever personality they want, especially in later gens where few characters actually have real backstories anymore.
Dani Moonstar and some of the New Recruits serve as this as well. As Dani only appeared in one episode, not much of her personality was fleshed out, and as such can serve as any personality type they want. Roberto only got a bit of characterization: A slightly egotistical 'jock' type, while Rahne got a snarky playful personality, both from just one episode, after that, they get no lines other than background appearances.
Ursa, Zuko and Azula's Missing Mom, from Avatar: The Last Airbender . She's of no small importance to the plot, given that she killed Azulon to save Zuko's life, but her only scenes are in Zuko's flashbacks and Azula's hallucinations. Fans have a field day discussing how accurate either of those are and what she was actually like. It doesn't help that the lack of a family reunion is the series' biggest unresolved plot thread.
Danny Phantom has a few, but none so popular as Star, or Star "Satellite" as the fandom has named her. She has some lines and establishing character moments, but in general, the Show portrays her as the blonde girl who hangs around the Alpha Bitch Paulina. Naturally, fanfic authors have a ball giving her Hidden Depths.
Take Care Bear is an unusual case. Her author created Take Care Bear as a custom bear, not one of the canon Care Bears, then inserted her into the world of the 1980s cartoon. This bear is a medical doctor, and her tummy symbol is an apple. Few fans knew that an official bear had the same name. Take Care Bear is a One-Shot Character in The Care Bears And The Big Sneeze, an obscure coloring book from 1987. This bear is a medical doctor, and her tummy symbol is an apple. (The 2004 toy line launched a new version of Take Care Bear.)
The similarity might be coincidence: "take care" is an idiom, and there is a proverb, "An apple a day keeps the doctor away." So it is plausible that the fan author, not aware of the coloring book, created a custom bear with the same name, job, and tummy symbol. The coloring-book bear has brown fur, but the fan-fiction bear has red fur. Fans may conflate two into one. Therefore, the fan-fiction Take Care Bear is an OC Stand In for the coloring-book bear.
Part of the South Park fandom will utilize this trope on minor classmates in the 4th grade class. Red was a popular choice in early years as she was the only one of the girls to have had no lines or discernable personality- up until season 6 and on. Kevin Stoley is the usual favorite these days, with fans attempting to give him an entire backstory beyond being the Star Trek/Star Wars geek and theorizing at length about his relationships and family.
In the later seasons of The Raccoons, an unnamed little raccoon girl made several silent cameos, and got one speaking role in 'Endless Summer', where we briefly see her talking with Bentley. She has seen use by many fan-fiction writers, usually giving her the name Annie Ringtail (with some writers naming her Penny). A great deal of these fan-fictions depict her as a love interest for Bentley, though how he returns the affection (if at all) varies by writer.