* Given the general quality of [[SturgeonsLaw 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 ''Franchise/HarryPotter'' fandom, the two most common victims of this are Dumbledore (typically flanderized from a well-meaning ObiWan into a sadistic ManipulativeBastard) and Ginny/Hermione (whoever's on the wrong side of DieForOurShip 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 ([[TheyJustDidntCare or don't care]]) how to keep her unique personality intact. Typically, these stories wind up revealing that Luna's BunnyEarsLawyer personality is a "mask" used to conceal a much more normal (read: ''boring'') character underneath.
** One of the [[{{SoBadItsGood}} best]] examples is of course ''MyImmortal'', where Dumbledore shouts "WHAT THE HELL ARE YOU DOING YOU MOTHERFUKERS!"
** Don't forget Ron, who gets turned evil on a regular basis, or at the very least his character flaws (including a bit of an inferiority complex) Flanderized into his only defining characteristics, often for the sake of DieForOurShip or because he's not ''special'' enough. He's not the TropeNamer for RonTheDeathEater for nothing!
** Snape tends to be either [[DracoInLeatherPants a poor, put-upon, sweet little fallen angel and one hell of a lover]] or a [[RonTheDeathEater cold, murderous sociopath who loves nothing more than raping and killing the students]]. Very, very few fanfics portray him as the complex, deeply flawed [[spoiler: but ultimately [[AntiHero sort-of heroic]] and noble]] character he is in the books. Usually the only personality trait of his they get right is the DeadpanSnarker aspect, and even that's pretty hit-and-miss.
** Even the people who are liked by the author aren't safe, just in the opposite direction. Hermione/Ginny (whoever's on the''right'' side of DieForOurShip this week) will be turned into a [[WallFlower weak-willed]] [[SuetifulAllAlong but beautiful]] arm candy who's only defining quality is being with Harry. Harry meanwhile, often becomes vindictive and heartless to any friend who the author doesn't like, assuming he doesn't become [[DesignatedHero an outright sociopath.]]
* ''Franchise/{{Digimon}}'': In addition to the usual MarySue-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 [[DieForOurShip 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.'
* ''Franchise/{{Transformers}}'': While there's a lot of HoYay there, make no mistake: ''Starscream really does want to kill Megatron.'' In fanfiction they have more of a SlapSlapKiss relationship.
* An [[DoubleSubverted interesting case]] happens in ''Fanfic/LightAndDarkTheAdventuresOfDarkYagami'' 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 ''Manga/DeathNote'', [[FoeYay LxLight]] fangirls will often do this to Misa for [[DieForOurShip 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.
* ''Anime/GundamWing'' fanfiction almost always does this to [[DieForOurShip 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[[note]]especially given that, as she is, [[ShipperOnDeck Duo ships Heero/Relena]][[/note]]. In fanfic, she's usually a catty, jealous shrieking harpy willing to kill to make Heero hers by force and blood. In canon, well, apart from anything else, her ImageSong is about how IWantMyBelovedToBeHappy and she'd be willing to leave him if he fell in love with someone else.
* ''Franchise/KingdomHearts'' fanfiction has the habit of taking the members of Organization XIII, the resident ambiguously evil group, and turning them into misunderstood people who only want to have hearts and it's the series' protagonist and his allies who are the real evil. The fics usually also gloss over the pesky "no hearts" part, despite this and the subsequent lack of emotion being the Nobodies' trademark characteristic in the games. [[spoiler: Granted, ''[[VideoGame/KingdomHearts3DDreamDropDistance Dream Drop Distance]]'' revealed that they do actually naturally develop replacements for their hearts over time, but this still does not excuse the general out-of-characterness that runs rampant in the fanworks.]]
* Perhaps one of the worst cases of character derailment ever seen is in the ''intensely'' [[LoveItOrHateIt polarizing]] ''Franchise/{{Pokemon}}'' fan fiction ''[[http://www.fanfiction.net/s/3119315/ Pokemon 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 KillAllHumans types who will gladly slaughter ''hundreds of thousands'' of civilians at the mildest coercion.
* A case of CharacterDerailment that may well surpass the above is in the ''LightNovel/HaruhiSuzumiya'' fic ''Fanfic/YouGotHaruhiRolled''. 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 [[TheDitz ditzy]] tendencies who is obsessed with [[LesYay Sasaki]], who is horrified at this. Not that the readers are complaining, mind you, for this is a CrackFic and as a result, [[TropesAreNotBad most of the readers actually consider this characterization to be]] ''[[TropesAreNotBad more]]'' [[TropesAreNotBad entertaining than Kyouko's canon personality.]]
* This trope is the whole premise of ''FanFic/FunWithLeonAndAshley'', where Leon is not only a {{Jerkass}}, but literally in love with himself and [[ADateWithRosiePalms spends an hour making love to himself]]. The result is hilarious, proving yet again that TropesAreNotBad.
* ''FanFic/LatiasJourney'' has several examples of this, most notably [[spoiler:Richie, Giovanni (after dying), Deoxys, and the fake ending's version of Mewtwo]]. In most cases this is justified by TheCorruption, in [[spoiler:Deoxys's]] case it's justified by RuleOfDrama (and the [[spoiler:Deoxys]] movie not being out when the story was started), and [[spoiler: the fake ending Mewtwo]] wasn't real to begin with.
** [[FanFic/BraveNewWorld 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 [[WesternAnimation/AvatarTheLastAirbender Zuko]] and [[NoMoreHeroes Shinobu]]. This could be attributed to the fact that she is a CompositeCharacter 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 TropesAreNotBad) is Bella Swan in ''FanFic/{{Luminosity}}'', where the normally annoying, wangsty and suetiful little strumpet is turned into... a rational person. Seriously, this is the catalyst for a ''Literature/{{Twilight}}'' fanfic that the haters genuinely love.
** Then, over in the ''Anime/NeonGenesisEvangelion'' fandom, there's ''FanFic/ShinjiAndWarhammer40K'', which derails Shinji from his canon, psychologically damaged self into Captain Awesome Badass [=McEpic=] von Powerfist, equal parts Kamina and Vance Motherfucking 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 [[spoiler:gets a bridge dropped on him.]]" He ''had'' no character, and was rarely even mentioned until [[spoiler:his death, which is sudden and unrelated to the plot.]]
* Despite being a wonderful author otherwise, Kana37 over at fanfiction.net tends to make {{Inuyasha}} a [[BastardBoyfriend soulless bastard]] who will just stand by when [[BigBad Naraku]] or [[FirstKiss Kikyo]] attacks [[SecondLove 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 TropesAreNotBad, there's [[http://harmonystuck.tumblr.com/ Harmonystuck]], a Webcomic/{{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 AUs in the fandom, and is generally considered an antidepressant to the canon.
* This happens in a lot of {{grimdark}} ''WesternAnimation/MyLittlePonyFriendshipIsMagic'' fan stories, most notably the horrid ''{{Fanfic/Cupcakes}}'' and its multitude of spinoffs, in which [[GenkiGirl Pinkie Pie]] turns out to be a [[SerialKiller murderous]] [[ImAHumanitarian cannibal]]. And then there's ''Sweet Apple Massacre'', which makes ''Cupcakes'' [[EvilerThanThou look downright tame]]. Not to mention ''Rainbow Factory'', in which the JerkWithAHeartOfGold Rainbow Dash turns evil.
** 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 MindRape!). 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 [[ConvenientlyCommonKink 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 enters 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).
** ''Fanfic/TheConversionBureau'' derails the Mane Six, hell most of the ponies, into hardcore xenophobes and [[FantasticRacism speciesists]].
** Of special note is Celestia, who often functions as the driving force behind most TCB plots. In just about every TCB or Anti-TCB story, her character is [[CharacterDerailment dragged mercilessly through the mud]], utterly shattering any semblance she has with the MLP show at all. Most authors HandWave this as Celestia has simply been a [[AlternateCharacterInterpretation very convincing actor]] when she needs to mislead people. Both sides show her as an [[ConflictBall increasingly less complex character]], who's [[MotiveDecay motives become shallow]] and seemingly evolve into a NecessaryWeasel to justify the TCB name. Many ACB fics simply force Celestia to [[JumpingOffTheSlipperySlope Jump off the Slippery Slope]], seemingly crossing as many {{MoralEventHorizon}}s as can be fathomed, to drum up the HumansAreSpecial message.
* This is because there's no way canon!Celestia would ever condone the TBC ponies' actions, or at least not in most TBC-verses.
*** A particularly infamous example is from ''Ten Minutes: Aftermath'', which purports to be a sequel to ''Ten Minutes,'' features both Michael and Celestia acting wildly out of character in contrast to the original. Whereas Michael was calm and determined in ''Ten Minutes,'' he is now ragingly angry, unsure of his own motivations, and pointlessly cruel. Celestia was shown in the original as aloof and arrogant; now she's TheMessiah. The bomb used by the human resistance has now also been declared a planet-destroyer despite there not having been any indication of this before. [[WriterOnBoard The author just likes them better this way.]]
*** In ''Recombinant 63'', [[Series/DoctorWho The Doctor]]--who fought off various [[AssimilationPlot similar schemes]] before--endorses Conversion as a legitimate means of preserving humanity. [[RuleOfCautiousEditingJudgment That is all]].
* There are ''plenty'' of ''[[Film/TheRockyHorrorPictureShow Rocky Horror Picture Show]]'' fanfics which derail Frank's character in favor of pairing him up with either a MarySue 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 [[TheHedonist 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 ClingyJealousGirl or a ditzy DamselInDistress 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 FoeYay 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 [[BrotherSisterIncest 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.
* ''Series/{{Torchwood}}'' fandom will often do this to Ianto Jones. For example, in stories that take place in the Year That Never Was (a ''Series/DoctorWho'' setting set between series 1 and 2 of ''Torchwood''), Ianto will [[SpotlightStealingSquad suddenly show up]] and [[PossessionSue defeat the Master often by himself.]] In canon, Ianto was a [[BattleButler genuinely badass character]], but so were [[TheCaptain Jack]], [[BewareTheNiceOnes Martha]] and [[BadassPacifist The Doctor]].
* [[InvokedTrope Done on purpose]] in ''[[http://www.fanfiction.net/s/8284907/1/The-Altomare-Ace The Altomare Ace]]''. The fic is intended as a {{Reconstruction}} of the FandomSpecificPlot of [[Anime/{{Pokemon}} Ash]] being betrayed and abandoned by his friends, so naturally his friends (with the exceptions of [[OnlySaneMan Max]] and [[TeamDad Brock]]) are derailed into {{Jerkass}} {{Arrogant Kung Fu Guy}}s. Whether the fic actually succeeds in its goal of reconstruction is [[BrokenBase contested]].
* The ''Literature/HarryPotter'' fanfic ''[[http://www.fanfiction.net/s/4605681/1/The_Real_Us The Real Us]]'' derails almost everyone:
** Since the author is a [[DieForOurShip Harmonian]][[note]]a Harry/Hermione shipper for those not in the know[[/note]] the whole Weasley family (save Arthur) gets treated horribly: Ron is a volatile idiot and Ginny is a whore; both of them are coached by [[EvilMatriarch Molly]] into slipping love potions to Harry and Hermione in order to separate them. Fred and George are greedy opportunists who don't care who gets hurt in their pranks, and it's as if Percy never got his CharacterDevelopment from the end of the last book.
*** That is not to say that all Harry/Hermione fans endorse this fanfic, because they do not. In fact, while a person may enjoy other works by this author, many people find this particular story to be painful to read, especially if they prefer to read Ron [[FireForgedFriends in a friendlier light]]... who just happens to find a girl for himself.
** Harry becomes an InsufferableGenius and Hermione is madly in love with him, wanting sex with him ever since they first met. ([[DoubleStandard And she is not punished for this, unlike Ginny.]]) Both only pretend to be friends with Ron, and perform [[LaserGuidedAmnesia Memory and Confundus Charms]] on him without the slightest provocation in order to deliberately deceive him for their entire time at school.
** Snape turns out to have been a NiceGuy all along, who likes Harry and only joined the Death Eaters because he was [[PlausibleDeniability completely unaware that Voldemort was a bigot]]. Did I mention that after the werewolf incident he became the [[SixthRanger fifth Marauder]]?
** [=McGonagall=] doesn't care in the least that two students constantly have sex with each other.
** Dumbledore outright encourages Harry and Hermione to keep up their deception of Ron.
** Most surprising of all, Umbridge is actually a genuinely nice person. It turns out that her BitchInSheepsClothing persona is just an act, and she idolized the Marauders when she was in school. (She is also younger than Harry's parents for some reason.) This of course serves to eliminate all conflict from the fic's retelling of the fifth book.
** Pretty much the only characters who escape relatively intact are the exceedingly minor ones, and the more important ones lucky enough to fall OutOfFocus.
* ''Disney/{{Frozen}}'' fanfic authors constantly see fit to derail the strong, valiant {{determinator}} Princess Anna, who managed to keep a strong sense of optimism even after years of isolation, into some sort of kicked puppy who is constantly suffering emotional breakdowns and is in desperate need of Elsa's affection. What Anna went through in the film can certainly be considered traumatic, but fanfic!Anna apparently can't go a second without something reminding her of her horrible childhood and leave her sobbing her heart out. Not only does this represent strong {{Chickification}}, but it also derails her selfless, others-first nature from the film and replaces it with a selfish AttentionWhore who constantly wants her sister to make things better, often actively disrupting her royal duties to do so. Usually this is done to make Elsa, who ''was'' very emotionally sensitive in the film, appear stronger, and thus put her in a dominant position when the [[IncestYayShipping romantic attraction]] breaks out between the two.
* One especially bad [[NotSafeForWork Nsfw]] ''Manga/{{Naruto}}'' story derailed Naruto's personality in the story. When he discovers that one of his fiances is pregnant, Naruto is overjoyed at the idea of being a father and having a family. A few chapters later he offhandedly mentions that while traveling the elemental nations, he never practiced safe sex and has (by his count) a few ''dozen'' children throughout the continent. He also tells another fiance that he doesn't really care if she sleeps around because he fully intends to do so himself.