Character Derailment/Fan Works

Examples of in  include:

Fanfic

 * Given the general quality of 90% of all fanfiction, it should go without saying that this happens a lot in fan-made stories, but even the good ones do this to their characters sometimes.
 * In the Harry Potter fandom, the two most common victims of this are Dumbledore (typically flanderized from a well-meaning Obi Wan into a sadistic Manipulative Bastard) and Ginny/Hermione (whoever's on the wrong side of Die for Our Ship this week), but other characters aren't immune to this either. Tragically, Luna Lovegood seems to be a frequent target of authors who don't know (or don't care) how to keep her unique personality intact. Typically, these stories wind up revealing that Luna's Crazy Awesome Bunny Ears Lawyer personality is a "mask" used to conceal a much more normal (read: boring) character underneath.
 * One of the best examples is of course My Immortal, where Dumbledore shouts: "What the hell are you doing you motherfukers!"
 * Snape tends to be either a poor, put-upon, sweet little fallen angel and one hell of a lover or a cold, murderous sociopath who loves nothing more than raping and killing the students. Very, very few fanfics portray him as the complex, deeply flawed  character he is in the books. Usually the only personality trait of his they get right is the Deadpan Snarker aspect, and even that's pretty hit-and-miss.
 * Digimon: In addition to the usual Mary Sue-related stuff (everyone loves Sue except for Suethor's least favorite character, who is now an ax murderer and gets hit by a semi... show me a fandom that doesn't have it) there are a few characters people like to pick on. Most prominently, Matt (Yamato) has never really been forgiven for winding up with Sora, so he's usually turned into an abuser and Sora falls into the arms of her One Twoo Wuv Tai(chi). Rika (Ruki) also winds up being nothing but a damsel in distress to be saved by [insert male character here.] It can actually be any of the male leads - the only consensus is that Rika's actual Badass canon persona won't do, she must do nothing but cry and be 'rescued.'
 * Transformers: While there's a lot of Ho Yay there, make no mistake: Starscream really does want to kill Megatron. In fanfiction they have more of a Slap Slap Kiss relationship.
 * An interesting case happens in Light and Dark The Adventures of Dark Yagami with Naomi. She's first introduced wanting to catch Kira for killing Raye, which is an accurate basic reflection of her character. However, it's revealed that it was actually Takada in disguise, and the real Naomi mainly serves as Dark's girlfriend.
 * And speaking of Death Note, LxLight fangirls will often do this to Misa for obvious reasons. Yes, Misa is not that smart compared to L and Light (and really, WHO IS?) but she is still pretty clever when she wants to be. Fanfic writers tend to ignore this and just make her a complete idiot (occasionally having her use Valley Girl speak to make her look even stupider) before ultimately killing her off.
 * Gundam Wing fanfiction almost always does this to Relena, Heero's canonical love interest, since there's more or less no possible way for Heero to get with Slasher favorite Duo while her personality is intact . In fanfic, she's usually a catty, jealous shrieking harpy willing to kill to make Heero her's by force and blood. In canon, well, apart from anything else, her Image Song is about how I Want My Beloved to Be Happy and she'd be willing to leave him if he fell in love with someone else.
 * Perhaps one of the worst cases of character derailment ever seen is in the intensely polarizing Pokémon fan fiction Pokémon Revolution written by Farla. In it, characterization is completely discarded. In two of the worst cases, Gary goes from asshat, egotistic rival to misanthropic sociopath. And Ash goes from loving but dim Pokémon trainer to hateful trainer with a raging vendetta. All the while, the pokémon themselves are derailed into Kill All Humans types who will gladly slaughter hundreds of thousands of civilians at the mildest coercion.
 * A case of Character Derailment that may well surpass the above is in the Haruhi Suzumiya fic You Got Haruhi Rolled. In it, the minor villain Tachibana Kyouko goes from a girl who thinks that her best friend would make a better deity than the title character, but still a very reasonable person who disapproves of her organization's excesses, to a Cloudcuckoolander with ditzy tendencies who is obsessed with Sasaki, who is horrified at this. Not that the readers are complaining, mind you, for this is a Crack Fic and as a result, most of the readers actually consider this characterization to be more entertaining than Kyouko's canon personality.
 * This trope is the whole premise of Fun with Leon and Ashley, where Leon is not only a Jerkass, but literally in love with himself and spends an hour making love to himself. The result is hilarious, proving yet again that Tropes Are Not Bad.
 * Latias' Journey has several examples of this, most notably . In most cases this is justified by The Corruption, in case it's justified by Rule of Drama (and the  movie not being out when the story was started), and   wasn't real to begin with.
 * The sequel's version of Dawn (yes, THAT Dawn) is so different from the anime's version that she might as well be a separate character. To clarify, she is essentially a mix between Zuko and Shinobu. This could be attributed to the fact that she is a Composite Character of Sadic from the aforementioned Latias' Journey and Dawn's Bunery in regards to her romance.
 * One of the best cases of this trope ever (showing that Tropes Are Not Bad) is Bella Swan in Luminosity, where the normally annoying, wangsty and suetiful little strumpet is turned into... a rational person. Seriously, this is the catalyst for a Twilight fanfic that the haters genuinely love.
 * Then, over in the Neon Genesis Evangelion fandom, there's Shinji and Warhammer 40 K, which derails Shinji from his canon, psychologically damaged self into Captain Awesome Badass McEpic von Powerfist, equal parts Kamina and Vance Motherf***ing Stubbs. This has been a wildly popular move, to the point where the page for this fic gets almost as much love as the one for Evangelion itself.
 * In Fandemonium, since several different writers are writing for several other different people's characters, personalities are often contradicted (except Hiro and Flames!) Most of all is Nightwolf, whose creator states "Nightwolf is mostly avoided being written about, although when he occasionally appeared he was written badly. I think that only twice in the entire series (not counting my episode) he acted sort of normal."
 * His entire role in Fandemonium was pretty much "hangs around for a while and then " He had no character, and was rarely even mentioned until
 * There are a few cases in Naruto Veangance Revelaitons, when the characters are not only derailed from what they were in canon, but are not even consistent with their original portrayals.
 * Sakura starts out as someone who tries to fight alongside the team, but doesn't contribute much (although mainly because she's overshadowed by the God Mode Sue main character Ronan). She later repeatedly gets kidnapped by Orochimaru and Madara, and in most of her fights, she cries in a corner while letting Ronan do all the fighting.
 * Naruto suddenly becomes jealous of Ronan after his return following his "death", eventually.
 * Original Character Taliana suffers from this. She's introduced as a love interest for Ronan who, despite being loyal to Madara, falls in love with Ronan. She then goes through a Heel Face Revolving Door, and alternates between having sex with Ronan and fighting him. Then, when the author broke up with the girlfriend she was based on, she attacks Sakura after, only to get beaten up by Sakura of all people. One chapter later, she converts to Christianity (which is a sign of being evil in this fic) and declares he always knew she was evil before brutally killing her.
 * Despite being a wonderful author otherwise, Kana37 over at fanfiction.net tends to make Inuyasha a soulless bastard who will just stand by when Naraku or Kikyo attacks Kagome, especially if it involves protecting Kikyo in some way, shape, or form. This is usually the premise for her Sess/Kag fics.
 * In an unusual example of Tropes Are Not Bad, there's Harmonystuck, a Homestuck AU in which the kids, trolls, Felt, and Midnight Crew are raised by a good version of Doc Scratch and every single evil character is nice. It's one of the sweetest A Us in the fandom, and is generally considered an antidepressant to the canon.
 * This happens in a lot of Grimdark My Little Pony Friendship Is Magic fan stories, most notably the horrid Cupcakes and its multitude of spinoffs, in which Pinkie Pie turns out to be a murderous cannibal. And then there's Sweet Apple Massacre, which makes Cupcakes look downright tame.
 * Then there's Pattycakes. Fluttershy is written as being so obsessed with having a child of her own that she psychologically breaks Rainbow Dash until the blue pegasus reverts to an infantile mental state, and in one ending totally destroys Scootaloo's mind for failing to comprehend the philosophy she never bothered to explain (Element of Kindness? Mo' like Element of Mind Rape!). Zecora is mentioned in backstory as having helped develop the mental regression drug, which fits with her herbal skills but not her actually possessing a functional brain. There's also a lot of ponies who are just sort of arbitrarily "into" age play, to the point of taking doses of the aforementioned drug - even ultra-prim-and-proper Rarity (who, if you suggested that she entering a state where she would periodically shit herself, would likely give you the same kind of expression you'd earn from the Pope if you visited him with pamphlets about converting to Islam).
 * There are plenty of Rocky Horror Picture Show fanfics which derail Frank's character in favor of pairing him up with either a Mary Sue or with whichever character they ship Frank with. Frank is often written as giving up his lustful ways (which he would never do because of his hedonistsic nature) to settle down with his "one true love". Often, this is the result of him "seeing the error of his ways" after he somehow hurts his "love interest" (About the only character he felt any remorse towards losing in the play is Columbia, and even then he's a complete Jerkass about it). Or, him being reunited with a "long lost lover" from his past.
 * This can apply to other characters as well. Columbia is either a Clingy Jealous Girl or a ditzy Damsel in Distress in the fanfics unlike the Harley Quinn-esque character she is in the play/movie.
 * Riff Raff is more often in not in fanfics portrayed as nothing more than a constant source of Foe Yay between him and Frank resulting in the duo becoming lovers. While it is true that Riff Raff did have some resentment towards Frank's treatment of him (hinting that he wanted Frank to like him), Riff's general attitude towards Frank is that of resentment and jealousy. Not to mention that Riff's canocial lover is his sister, Magenta.
 * Speaking of Magenta, most Frank/Riff Raff fanfics either portray her as a manipulative and abusivise sibling towars Riff Raff or she simply doesn't exist. In the play/movie, she shares a very close bond with her brother and even helps him rebel against Frank.