Luanne started out as a somewhat promiscuous teenager who didn't take crap from anyone and, despite being Book Dumb, had some intelligence (the episode that fully introduced Hank's father, Cotton, revealed that Luanne can fix a car, despite Cotton's "Stay in the Kitchen" views on women). Later episodes turned her into a shy moron who was afraid to stand up for herself (and when she married Lucky, her naivete and lack of intelligence were greatly exaggerated to childlike levels). The promiscuity, however, was given a proper episode (where Luanne joins a group of promiscuous people who want to be born-again virgins) when it disappeared.
Bobby Hill was similarly much less effeminate in his early years. It could be argued as Character Development though if you didn't take well to his original characterization as a "Well Done, Son" Guy who's slow, awkward, and, in the words of Hank Hill, "not right." Sure he got effeminate in later episodes, but he got also got some charisma and charm, even if it was limited to being a class clown. He also went from a shy, reclusive fat kid to someone who has friends, a girlfriend, is sure of himself, and has proven to both Hank and Cotton (the episode where Cotton takes over his military school, and goes one day longer in "The Hole" than Cotton managed) how much of a badass he is. All while not changing himself to fit how they think he should be.
On the other hand, some consider Cotton to have undergone a case of derailment during his final appearance, which retcons his grudge against the Japanese despite him having forgiven them in an earlier episode, talked out of his scheme to spit in the Emperor's face by his illegitimate son Junichiro. This gained the episode criticism from some viewers since while Cotton was not well-liked, the way he forgave the Japanese was considered development.
Peggy originally started out as a well-meaning wife and mother, as well as being pretty sociable and having a fair amount of friends to hang out with, and being rather fluent in Spanish. As the series went on though, she slowly became more and more narcissistic, and while still caring for Hank and Bobby, was not afraid to call them out if they did something that even slightly annoyed her, while her group of friends was slowly narrowed down to Nancy and Minh, and her Spanish became so terrible that it was used as a plot point once to get her out of a court trial.
Hank had also undergone Derailment as the show went on. During the first few seasons, he was shown to enjoy Rock and Roll and practiced guitar at times and open to new things, despite his usual staid and conservative attitude. By the end, he was a stuck-up tight-ass who was so old-fashioned you'd think he was Amish and views things out of his comfort zone with contempt and disdain.
This is addressed in the episode "Get Your Freak Off", where Hank discovers Bobby has been listening to foulmouthed gangsta rap. He initially encourages Bobby's interest in a boy band he considers clean-cut and safe- until he, Bobby, and Bobby's girlfriend go to a concert and begin dancing provocatively. Bobby's girlfriend has ridiculously permissive parents, which leads to a situation Bobby and even his girlfriend weren't comfortable with, and Hank comes to the rescue and teaches the kids to play an age-appropriate game. While initially seeming to uphold Hank's uptight views as the best way to raise a child, it does show how extreme (and boring) Hank had become, and the ending shows him lightening up quite a bit.
Transformers: Beast Machines did this to a majority of the cast because the writers were explicitly instructed not to go back and watch through the previous series because they didn't want it to be "too continuity-driven." Optimus Primal from a competent and inspirational yet down-to-earth and approachable leader into a spiritual guru who bordered on fanaticism. Dark Action Girl Blackarachnia became the Satellite Love Interest deprived of her boyfriend. Megatron became obsessed with eliminating organic, individuality, and hated Beast Modes without explanation, though he wound up mainly reverting to his old self towards the end of the series. Knight in Shining Armor Silverbolt becomes bitter, cynical, and guilt wracked which was supposed to be explained by their time as Jetstorm, except they had their old personality when briefly freed from it.
Rhinox is possibly the biggest derailment of them all. Once he's brought to the surface and put in control of Tankor, it is revealed that he agrees with Megatron. From the level-headed, spiritually-minded second in command of the Maximals, who actually enjoyed Earth's natural environment, to a self-stated Beast Mode-hating supporter of Megatron. It was intended for Rhinox to be a lot more reasonable about it, explaining exactly why he thought this now, but Richard Newman's delivery of the lines made him sound like he had gone insane. A lot of fans pretend that the Vehicon programming was still affecting him.
Many of the characters in Thomas & Friends have suffered from character derailment to varying degrees:
Edward, previously an old, kind, and wise engine, became younger and ruder in later seasons.
Percy went from a mischievous Cloud Cuckoo Lander to a naive, forgetful moron who can't pronounce simple words.
Toby was a kind and confident Cool Old Guy but later became a wimp with self-esteem issues.
Skarloey and Rheneas, previously two of the oldest and wisest engines on the island, are now childish and afraid of everything (from thunderstorms to the incline yards to the wharf), while Sir Handel is turned from the jerkiest of the narrow-gauge engines to the kind voice of reason, effectively swapping personalities with the former two engines.
Some derailments have been taken a different way in the CGI series, either reversed back, exaggerated, or taken a different way altogether. Skarloey is back to his more mature and no-nonsense persona, while Edward is kind again, albeit to a fault, unable to say no to others and easily dissuaded from his work. Thomas interchanges between his older and newer persona to erratic extremes while Toby's meekness has reached Fluttershy/Chuckie Finster/Luigi levels, being afraid of nearly anything that moves.
Diesel 10, who currently provides the page image, is a very unique case of derailment. In his debut in Thomas and the Magic Railroad, he was a murderous monster who was out for blood, wanting to completely and utterly destroy the steam engines. In his unexpected second appearance in Calling All Engines, he's somehow become nicer with almost no given explanation, and quite willingly helps Thomas repair the new Sodor Airport. In his defense, it was related more to necessity since a decline in tourists brought in by the airport in question would've spelt doom for the entire railway, including him. Even so, he doesn't display any sort of gruffness or malice, and again, does it with a big smile on his face. However, when brought in for the CGI series, he became more of a mix of the two. Whilst he certainly wasn't nice like he was in CAE, it wasn't up to his TATMR persona, as he's now become more of a Well-Intentioned Extremist in that he wants his fellow diesels to receive more praise and attention than what they have, believing that they've become overlooked in favor of the steam engines, and while he certainly doesn't like them, he does realize when he's gone too far whenever he deals with them. It has given him more of a personality though, as some felt that his TATMR appearance was more of a Generic Doomsday Villain than an actual character. His appearance in Day of the Diesels returns him to his original villainous persona, where he and his band of diesels seize control of the Steamworks.
Pam in Archer became a completely different character as the series went on. Starting off as a pleasant but lonely and depressed HR rep, she turned into a sex-crazed, drug-addicted complete loon. Though as always, Tropes Are Tools, as many fans enjoy this version of Pam much more.
Stan Smith went from a liberal and mature if somewhat overreactive man to a complete psychopath in later episodes, although he occasionally showed signs of his later characterization in the golden seasons. However, he still retains a moral attitude, even in later seasons, but still acts foolhardy.
Steve Smith also has this as he went from a nerdy and laid-back boy who has a lot of common sense to a socially-inept Spoiled Brat, but like some of the characters on the list, he actually does see the error of his ways.
Roger also belongs on the list as at the start of the series, he is seen as a sensitive if somewhat snarky pushover but transforms into a total sociopath willing to kill, manipulate, emotionally scar, and ignore others for his own personal ambitions. Stan, despite being on the list himself and does things no different or worse compared to Roger, is disturbed by his antics. He also went from being a clandestine shut-in, with entire episodes revolving around the Smith family's attempts to conceal an alien from the public and the government, to quickly becoming so adept at dressing up in his costumes that his visibility in public is no longer implied to be dangerous.
Johnny Bravo was, in the first season, a largely unflappable jock who tried a little too hard to impress the ladies, and who wasn't too bright but could occasionally be quite clever. In seasons two and three, when creator Van Partible left the show, in a case of Depending on the Writer, he turned into a childish, idioticJerkass who Screams Like a Little Girl. He reverted to his older characterization in the fourth season when Partible made his return to the series.
Ben 10's Darker and Edgier sequel, Ben 10: Alien Force, has Ben and Gwen exhibit different personalities than before, but that's excusable since it's been five years and they've matured. However, there is NO explanation for the change in Kevin, who not only has suffered massive Badass Decay but is now a good guy for poorly constructed reasons and shows no signs of the sociopathic cruelty he did in the original series. Now all he can do is be blunt, demonstrate The Worf Effect, drive them around in his beloved car, and occasionally thrive off of his reputation from back when he was an actual threat. Both Ultimate Alien AND Omniverse made attempts at Character Rerailment, but the fanbase remains perpetually divided over how well either of them went.
Vilgax went through some change as well; Alien Force portrayed him as Lawful Evil (though it lasts only one episode and it's strongly hinted he was actually posing as such for his own advantage; in later episodes, he has no problem breaking laws); later, Ultimate Alien portrayed him as a Manipulative Bastard, which, while being some derailment in a way, was actually good and ended up being an Author's Saving Throw after the Badass Decay he had suffered in Alien Force.
Also, Gwen has become less "mature and responsible" and more "perpetually serious and angry".
Word of God for Ben 10: Omniverse is that this derailment actually held Charmcaster up in appearing on the show throughout the first half, as it was very difficult to think up a Character Rerailment that would make enough sense after it. They ultimately opted for a Retcon of the above event through a complete lack of mention in the show and adding in dialogue that directly contradicts it if it were to have happened.
The entire franchise has a very complex case with titular protagonist Ben Tennyson; he started out as an immature brat, an Idiot Hero, and a Crouching Moron, Hidden Badass with a major Chronic Hero Syndrome, before being evolved into a more mature and competent character in Ben 10: Alien Force. Come season 3 of Alien Force, he Took a Level in Jerkassand in Dumbass due partially to an Acquired Situational Narcissism and partially an attempt from writers to make him more like his original self. Ben 10: Ultimate Alien tried to balance his maturity and immaturity, with more or less successful results depending on the episodes and story arcs. Finally, come Ben 10: Omniverse, the character has seemingly lost most of the maturity and competence he gained through the Alien Force/Ultimate Alien era and ended up all the way back to his characterization from the original series.
This ended up happening to Chloé Bourgeois. While she started out as an antagonistic Alpha Bitch, she was shown engaging in various Pet the Dog moments across the show, and various events in season 2 saw her become nicer and more heroic overall, complete with a Freudian Excuse being revealed for her nastiness. However, from the season 3 finale onwards, she's not only lost all her Character Development, but has arguably become worse than she was originally; she no longer has any moments of kindness, she's gone from just being a mean girl to actively working with the Big Bad, and her Freudian Excuse is being completely ignored in favor of a "she was just born evil" narrative.
Kim got hit with this hard in "Derision". While he did have a couple of Jerk Jock moments in season 1, for the most part he'd grown out of that attitude in favor of being characterized as a Hot-Blooded but kind Dumb Jock. "Derision", however, depicts him as having taken part in a cruel prank against Marinette in the past; in the present, he sees her trauma over it as an overreaction, and he calls the above-mentioned Chloé the prettiest girl in the world while he's talking to his girlfriend Ondine. Be it idiocy or outright maliciousness, it's still a far cry from how he'd been characterized in the past few seasons.
SpongeBob himself started out as a naive, yet kindhearted guy who cared about everyone no matter what, and had the innocence of a child. He later gets derailed into an obnoxious, Manchild who pesters everyone he knows to play with him. He is often careless of others' safety and cries and screams when his "friends" aren't around. He can even be a Jerkass once in a while, most notably in "A Pal For Gary".
Patrick got progressively dumber as the series went on. He goes from average intelligence in the first season to The Ditz in seasons 2 and 3 to virtually brain-dead in season 4 and onwards. He also became more of a Jerkass on occasion.
Plankton however started out as a competent, aggressive, and maniacal villain. Once The Movie rolled around he lost much of the latter in favor of being either too obnoxiously happy with the few moments he seems to succeed, or constantly moaning and crying about what a failure he is. He's also lost a lot of competence, to the point that his Deadpan Snarker computer wife Karen often has to tell him how to do every basic step (even blink) with Plankton often taking credit for her suggestions.
Karen herself was always a Beleaguered Assistant but was initially genuinely loyal and caring to Plankton despite her snarky qualities. Following the UnCancellation, she has seemingly lost all sense of enthusiasm and openly vents her utter contempt for Plankton, placing him into high order Henpecked Husband territory. In seasons 11-12, her main role is to make jokes about how stupid Plankton's plans are (such as going off to watch a movie or even unplugging herself so she doesn't have to hear it).
Squidward went through this twice:
In the first seasons, Squidward was portrayed as a Jerk with a Heart of Gold who still likes SpongeBob despite his attitude. His job at the Krusty Krab is something that he's generally apathetic with, doing the bare minimum possible. Seasons 6-8 and 12 onwards portray Squidward as either afraid of SpongeBob and Patrick, or outright malicious in actively wishing for SpongeBob's death. He also gets much more hateful about his job, viewing it as an awful form of torture rather than just something he needs to do for a living.
Seasons 9-11 are an utterly bizarre turn for the character, where Squidward randomly becomes prone to childish fits. He freaks out over not getting ice cream, pantses Mr. Krabs, covers himself in dirt and dust while screaming about bubble baths, throws a tantrum after losing a board game. It's quite jarring to see one of the more serious characters on the show leap out of his bathroom window, then run around town naked and use his tongue as a helicopter while warning everyone about "aliens from Mars." This change goes completely unexplained, but would thankfully be somewhat reverted by season 12.
While the fanbase seems completely divided whether Daffy Duck's Flanderization during the 1950s from a bombastic Cloudcuckoolander into a JerkassSmall Name, Big Ego counts as derailment or development, most agree it crossed the line during the DePatie-Freleng Enterprises era shorts, where he was evolved into a humorless, Faux Affably Evil villain for Speedy Gonzales. One could take it as a double-sided coin, as pitting Speedy against a more conniving villain did at least allow him to come off as more sympathetic and fallible more often than he did againstSylvester, though it still would have been a swifter transition with a more fittingly-callous character.
He's even worse in The Looney Tunes Show: while he does have his Cloudcuckoolander and Jerkass moments, he is a total dumbass who can't read or write, can't answer even the simplest trivia questions correctly, doesn't understand the concept of stealing, doesn't know the concept of time, etc.
Though, really, this change was happening even in some of the Looney Tunes shorts of the late-1950s and early-1960s. Witness Bugs' cartoons with Daffy Duck during that time period: he usually played the more sensible Straight Man to Daffy's hyper-idealistic loonatic persona.
In the Wile E. Coyote and the Road Runner shorts in the DePatie-Freleng era (most notoriously by Rudy Larriva) the Road Runner actively fights back against Wile E. Coyote, something he rarely, if ever did in the original Chuck Jones shortsnote Jones made 9 rules for the Road Runner shorts, one of which being that the Road Runner cannot harm the coyote except by going "beep-beep". This continues into 2003's "The Whizzard of Ow" and the CG shorts from the Looney Tunes Show, which kept the Slapstick nature of the franchise, unlike the show proper, such as "Silent but Deadly".
Bloo in the Pilot of Foster's Home for Imaginary Friends and Bloo during the main show are practically two different characters. Bloo in the pilot and some of the earlier episodes is a friendly and well-meaning but mischievous imaginary friend who likes to have fun. In the rest of the series, he's just a Jerkass who will do anything to get what he wants and loves being stupid and breaking the rules.
Cosmo is the show's main offender of this. It was established in the first season that he ran away from home to marry Wanda. In another episode, he broke down crying from having to be away from her overnight. It was very clear they enjoyed their relationship. Then, suddenly, in seasons four and five he was drooling over every attractive woman that came onscreen, repeatedly calling Wanda a nag and more, and acting like she had Bound and Gagged him at the altar. Hell, the original Oh Yeah! Cartoons shorts of Fairly OddParents had him be a lot different. Compare the calm, deeper-voiced Cosmo who was Wanda's equal intelligence-wise and actually seemed to share a loving relationship with his wife. Now, compare that to the sociopathic Manchild with the screechy voice portrayed in the Fairly Odd Parents series and see how far he fell. It's rather handy, you can tell his intellect by his voice pitch. Deep/slightly high voice, reasonable degree of smarts, really high voice, no smarts.
Vicky went from just using tapes, recordings, personal belonging-destruction, and other tiny bits of humiliation to torture Timmy to being armed to the teeth with chainsaws, maces, and other over-the-top weaponry and cackling maniacally all the while. This derails Vicky from as far back as the first episode, where she clearly shows she'd never want Timmy to suffer any serious physical harm, let alone be the cause of it, because her well-paying job depends on him being kept intact.
Even Timmy himself technically counts. He started off as a typical frustrated 10-year-old, a bit of an outcast, and occasionally selfish. He always learned his lesson if he made a mistake, though, and only made wishes whenever they were issues crucial to his life. Then later he became more self-centered and wishing for stuff he wanted. Though season 6 and onward more or less reverted Timmy back to how he was in the first season.
Timmy's parents are an interesting case. When the series first started, they were portrayed as tired and overworked from their jobs, but they still genuinely cared about their son. Their biggest crime was that they were more than willing to lie to Timmy so they could get time to themselves; however, they both seemed to learn their lesson in "Abra-Catastrophe." But as the series went on, we learn that Timmy's parents were expecting a girl (hence his pink hat), and in the infamous "It's a Wishful Life" episode, it's shown that Timmy's parents would have a child actress for a daughter and would be rich if Timmy had never existed. Then, both of Timmy's parents started making comments about how they could rent out his room if he went to military school (even jumping on a trampoline at the thought of it), and even casually admit that they sometimes forget to feed Timmy. And that's not even getting into how Timmy's dad, like Cosmo, was turned into an idiotic Manchild. Sure, Timmy's parents have been able to deliver the occasional Heartwarming Moment in later seasons, but there's no denying that somewhere along the line, they went from busy, albeit genuinely caring parents to borderline Abusive Parents.
Trixie Tang was initially written as a girl who was popular and spoiled, but had a good heart deep down, and was more than she appeared to be. Jump to "Just the Two of Us" where she's extremely shallow and vain, and can't seem to go even a second without someone telling her she's pretty, otherwise she'll completely lose her shit.
Tom and Jerry's movie is mostly infamous due to having the titular duo (who are The Voiceless and mortal enemies) talk, sing, and be friends.
The Character Derailment actually happens within the movie itself, as Tom and Jerry spend the first ten minutes of the movie as their normal, mute, bitter enemy selves (though Tom doessave Jerry from the collapsing house, it feels more like one of the Enemy Mine moments they'd occasionally have in the original shorts) until Puggsy the dog convinces them to be friends. For about a minute, and their first bits of dialogue, they seem to cling desperately to their characterizations as enemies, as well as engaging in some admittedly pretty funny lampshading of the sudden shift:
Tom/Jerry: You talked!!
Tom: Well, sure I talk! What do you think I am, a dummy?
Jerry: You said it, I didn't!
Tom: Ah, you little pipsqueak! I oughtta — *pause* Hey! How come you never spoke before?!
Jerry: Well, there was nothing I wanted to say that I thought you'd understand... and there STILL isn't!
This goes back to 1975 when Hanna-Barbera rebooted Tom and Jerry for ABC Saturday mornings. They started out antagonistic in the first episode "No Way, Stowaways," only for them to shake hands at the end. As the series went on after two or three episodes of going at each other (without the old films' violence), they remained bosom buddies.
Scott Summers/Cyclops on Wolverine and the X-Men (2009). In his origin episode Breakdown Cyclops, who has been depressed and ineffectual since losing his girlfriend Jean Grey, is revealed to have been extremely stupid and clumsy when he was first recruited to train as an X-Man. This apparently changed when Jean came into his life and "took the pain away" as Emma Frost, the rival love interest, put it. This is a big turn from the comics, where Cyclops was always a good, diligent X-Man, even as a student, where his primary problem was that he had weak social skills. This alternative characterization, however, embraces the stereotype that he is nothing without his long-time girlfriend, while not showing anything suggesting he was ever a competent heroic leader.
Everyone (excluding Nightcrawler) was derailed to some extent, becoming massively incompetent just to make Logan's aspects all the more apparent.
Granddad on The Boondocks: In the first season, he was one of the two straight men, generally went his own way and couldn't stand his grandsons insufferable Huey or no-good trouble-making Riley and didn't hesitate to take off his belt and give Riley an ass whooping whenever he had it coming. He's now a full-on Jerkassabusive grandparent who wants to seem young and hip, obsessed with money & "bitches", and blindly follows along with whatever dishonest schemes Riley has planned to get rich quick, over Huey's loud and sensible objections. Granddad over the course of 3 seasons came to embrace all of the black stereotypes Aaron McGruder has been making fun of (when it was just Riley).
Ironically, the one sillier trait Grandad had from the off, that he would greatly exaggerate, or outright lie, about his role in the American Civil Rights movements (he once showed up late to a march because he had to go get a raincoat), would also be submarined by later seasons. It turns out that Grandad was actually a member of the Freedom Riders, who rode buses into the Deep South to protest the southern state's discrimination against black people. While it only was he got on to the wrong bus by accident, early season Grandad would surely have been eager to brag about his role, conveniently omitting it was entirely unintentional.
Characters in the PBS show Arthur would fall victim to this in later seasons. It is especially poignant with episodes written by Dietrich Smith, though it varies depending on who writes. This is especially problematic, particularly among older fans, as Arthur is that kind of show with an established continuity and set character personalities.
Seasons 16 and 17 were full of them. In "Flippity Francine" and "Francine Speaks Up" Francine is scared of attention despite loving attention and always wanting to be the star for the past 15 seasons. In "Read and Flumberghast" the entire third grade class is made way more gullible than ever. In "Opposites Distract" and "Prunella the Packrat" Arthur is a Neat Freak who throws everything he feels is old away, while in the older episodes, he makes huge messes and is an extreme pack rat who won't even throw a popsicle stick away. The most extreme example is in "So Funny I Forgot to Laugh", where Arthur becomes a bully and Francine gets angry at him despite past episodes that revolve around bullying showing the bully (who is usually Francine) doing far worse things than what Arthur did. It also portrays Sue Ellen as someone unable to stand up for herself when Arthur bullies her, while in the first season, she was the only student who could intimidate Binky.
In almost a reversal of the aforementioned Bloo example, Rufus and Amberley of The Dreamstone, despite always being cutesy and cheery protagonists in contrast to their enemies, had some visible cynical traits and flaws in their own right in the pilot (Rufus was a Cloud Cuckoo LanderBadass Adorable while Amberley was a moody tempered but thoroughly helpful Vitriolic Best Bud) and had almost an equal focus in sympathetic spotlight and humor as the villains. In the majority of later episodes, the two became ridiculously juvenile and ineffectual, to the point their interaction was often extremely uneventful until the Urpneys attacked them every episode.
Their dynamic also derailed in some form due to the Urpneys decaying. In the first episode, the Urpneys were pitiful unwilling bumblers, but they and Zordrak were still effective enough to put the heroes in genuine mortal danger, allowing Rufus to come off as sympathetic against them. In most later episodes, they are completely ineffectual and doomed to failure from the start, with the heroes gaining a smug awareness of this and often taking fun out of tormenting the Urpneys just to ease the monotony of their routine schemes. In one episode, the Noops outright retreated in terror from the Urpneys because they had become too close an actual threat, nearing them more as self-righteous Miles Gloriosuses who are only willing to bully harmless targets.
At least some attempts were made to remedy both cases of Derailment throughout Seasons Three and Four, giving the Noops more Character Checks to their old personas and reverting them to competent but pragmatic heroes against the Urpneys (along with giving the villains a more Not So Harmless motive so that the heroes came off as genuinely defending themselves).
Zordrak, was originally a calculating and genuinely intimidating Big Bad who took part in the scheming and entered the fray every once in a while as a more imposing threat. After Season One, Zordrak gradually devolved into an ineffectual Bad Boss who had no importance outside sitting on his throne and yelling at his Urpneys to make a new plan to take the stone. Similar to the heroes, he gained a partial remedy in the last few episodes, albeit to a far lesser degree.
Everyone in the Watership Downseries adaption. For starters, Hazel is suddenly in love with Primrose after about two episodes, and Hyzenthlay was completely replaced altogether. Bigwig, in both the book and movie, was a bold, strong, and knowledgeable fighter. Here he's turned into nothing more than a complainer and follower to Hazel, who doesn't know anything in war strategy. Pipkin, previously nervous, but extremely loyal, is turned into a Cheerful Child. Even Woundwort - the genocidal, slave-driving Big Bad who eats hawks for breakfast and in the book contemplates how some day he hopes to meet a stoat so he can kill it - had a period in the series where he tried to stop the war with Watership Down and openly showed remorse for what he'd done. And the list goes on.
While the Character Derailment from the book is fairly rampant, the show has a second round at the start of the third season, when the show also had a drastic change in art style and writing. Woundwort is again derailed the most. The season transition happens in the middle of the final battle with Efrafa, leading to a moment where Woundwort realizes the error of his ways and tries to stop the war. He's then struck by lightning and the warren collapses. He digs himself out afterwards and declares that he will have become a force of destruction, rather than simply a totalitarian dictator.
Takua was originally an adventurous and brave character who valued duty and helping others (even if he had just met them) above all else, and who would be among the first to volunteer for dangerous quests. In the movie, an easygoing slacker who shoves his duty on others, cowers in the face of danger, and abandons his friend to do his work for him (a friend he tricked into believing it was his work in the first place). It takes the destruction of several villages and seeing his heroes fall for him to realize this is no time for goofing off. Very striking, as prior to the movie's events, he was hailed as a war hero.
Takua's friend Jaller goes from a serious and mature military leader-type to an almost childlike, somewhat insecure, and shy-around-girls nerd. In the preceding story, he snapped at Toa Tahu when he was losing confidence in himself and defeated the extremely dangerous Pahrak-Kal by simply walking up to it and grabbing its power-giving Krana. In the movie, although still not a wimp, he doesn't showcase any of this militarist sternness and simply lets Takua bail out of their mission.
Onua, the wisest and most level-headed of the Toa, who always chooses the most effective yet least risky path in every battle (it's written in his official bio), is portrayed here as a slow-minded brute who puts his partners in danger with his actions, and causes the destruction of his hometown, and almost the death of his entire team. Thankfully, this portrayal didn't stick after the movie's saga.
Hahli's supposedly shy (which even the character explorer on the DVD points out) but very friendly, especially towards Jaller, but the movie introduces her as a tough chick who teases Jaller with a much more "sassy" attitude.
Lewa on the other hand was usually portrayed as a juvenile, hyperactive loner with a very sarcastic sense of humor and often held little respect for the other Toa — in the movie, he comes off as a spiritual hippie who actively volunteers to group together with others and seems to be one of the most mature of the Toa.
Vakama goes through considerable change between the second and third movies. At first, he's an incredibly insecure would-be leader who then learns to trust himself, his visions, and not to be afraid to stand up for himself. The following film introduces him as an arrogant, overconfident, reckless oaf who leads his team into a trap despite absolutely everything around him, including his visions, the warning words of his teammates (and a villain he met in the books between the two films) and the threatening signs found all over the ruined city, telling him to act cautious.
Scrappy from Scooby-Doo. The very first episode with him in it, he played a major role in incapacitating the bad guy, but as time went on, Moral Guardians, thinking that Scrappy might be a bad influence on kids, had him being toned down until the point where he actually shows fear in The New Scooby-Doo Mysteries. Quite a few fans thought this was an improvement, though others who'd liked Scrappy before disliked how out of character it was. He still had his moments though, and picked up the ball again in The 13 Ghosts of Scooby-Doo and the Superstars Ten Movies.
Stanley has a minor version: For the first few episodes, Lionel acted like your standard Big Brother Bully. Then, as if overnight, he become a Cool Big Brother.
Batman: The Brave and the Bold did this on purpose in the Grand Finale. Bat-Mite is using his Reality Warper powers to make the show Jump the Shark and get cancelled, and the last of his changes is making Batman use guns. When Ambush Bug points that Batman Does Not Like Guns because of his parents' murder, Bats finally starts listening to him and believing that someone is messing with their universe.
Andy Panda: In 1944, director Shamus Culhane wanted to try out a new, redesigned Andy that was a total Jerkass instead of a Mickey Mouse-type everyman. The new Andy was hated by Walter Lantz and audiences alike, and so he was dropped after just one cartoon, "The Painter and the Pointer", in which Andy threatened to kill his dog if the dog didn't pose for his painting, and to make sure he didn't move, Andy rigged a shotgun to shoot the dog if he moved from that position.
Quite possibly the character of Sheen from The Adventures of Jimmy Neutron, Boy Genius and its Spin-OffPlanet Sheen. In fact, the derailment is due to the Spin-Off. While Sheen could be insensitive and oblivious to others, it was clear he cared about his friends and family in the Jimmy Neutron series. However, in Planet Sheen, he is utterly indifferent about his family and friends to the point where he doesn't even attempt to repair the damaged rocket to go back to Earth.
The Warden of Superjail! losing his sadism after the pilot and gradually becoming more of an outright Manchild (as opposed to Psychopathic Manchild) can be seen as this for some fans, although it's a highly contested example. He also winds up no longer being someone to be feared, but instead be beat up and easily taken advantage of, and have his obliviousness and naivety cranked up.
While it may have redeemed her in some eyes, the Mistress' overall portrayal in "Stingstress" was ridiculed. Suddenly, the Mistress was revealed to beincrediblyeasily fooled by the presence of a man she found attractive, was portrayed as lovesick for him, and weepy and emotional at being pushed around. Then her change at the end as "Hippie Mistress", revealing that she only needed one good experience of sex to loosen her up (and realize that she didn't need any men or the Superjail), was further seen as this for some (especially for the dejected shippers of Warden/Mistress).
The Twins experienced this in season 2's "The Budding of the Warbuxx", due to it putting way too much emphasis on how alien and disgusting they were. Though in general, the outright revelation of them being aliens is taken this way by a portion of the fanbase (as it also coincided with them either being put Out of Focus or having their vulnerability and foolishness emphasized more than their other traits).
While Lemongrab of Adventure Time was always a huge Base-Breaking Character and even the staff of the show had different opinions of him, after his first appearance the show seemed to lean towards the idea that he wasn't truly evil, just really messed up. In his story in the third collection of character shorts, he seemingly devours his twin brother. In his next appearance, we see that he's become a tyrant who abuses his now maimed brother, and torments his lemon children when he had previously been portrayed as a loving parent. At the end of the episode, he fully eats his kinder brother and loses any of the redeeming traits he may have had before. The next episode continues to show him keeping the lemon kingdom in misery, torturing his subjects for fun. He's ultimately torn apart, and then stitched back together, the characters suggesting that now maybe he'll be better. The only thing more sour than Lemongrab is the taste the whole thing left in his fans' mouths.
Finn in the episode "Too Old." In the previous episode, he breaks up with his girlfriend, so in this one, he tries to flirt with his previous love interest, Princess Bubblegum, while largely ignoring or not focusing on the crisis in Castle Lemongrab. In the past, love had clouded Finn's heroic senses, but this episode took it above and beyond.
In Hyperion Studios' TV series of The Itsy Bitsy Spider (1994-96, based on their short subject which played with the feature Bebe's Kids), Leslie was a sweet, introverted Bespectacled Cutie in the first season. In season two, she became rather abrasive to her schoolmates and friends, showing occasional signs of emotion.
Randy Marsh's gradual transition from calm and responsible father to creepy immature Manchild is seen as an example of this by many. Definitely a case of Depending on the Writer, though, as some episodes still portray him as a pretty normal and responsible father.
Kyle also gets derailed in some "Kyle vs. Cartman" episodes. He's mostly somewhat snarky but otherwise nice and definitely a positive character compared to Cartman, but some episodes have him be a rudeKnight Templar who is as bad as Cartman. This characterization of Kyle is also used in other episodes, such as "Douche and Turd", where he uses Puff Daddy to intimidate Stan into voting, and "Fatbeard", where he is willing to send Cartman's entire Pirate crew to their death in Somalia.
Jimmy Two-Shoes underwent in a retool as it transitioned into its second season and furthered itself from its Darker and Edgier pilot concept, meaning the entire main cast was hit by this.
Jimmy himself. In Season 1, he was simply a cheerful and fun-loving boy who wanted to make friends with and put smiles on everyone in Miseryville, and although he was childish and naive, he could be surprisingly clever and even the Only Sane Man. The Jimmy of Season 2 is a manic and hyperactive Cloud Cuckoolander who never has a single moment of intelligence or sanity, only treats Beezy with any real respect, and is totally callous to the mayhem he causes in his thrill-seeking. He even starts showing attraction to girls in several episodes despite the fact that one of his defining traits is that he is Oblivious to Love.
Beezy in Season 1 was a Jerk with a Heart of Gold who was supportive of Jimmy, very mellow and easygoing, well-intentioned, and cared a lot about his girlfriend Saffi despite his stupidity and hedonism; the entire point of his character was that, underneath his many flaws and vices, he was a better person than his evil father Lucius. The Season 2 Beezy, on the other hand, is basically a big and fat version of Lucius with a gross case of Small Name, Big Ego and a total jerkass who treats everyone around him like garbage (to the point where the people of Miseryville hate him as much as they do Lucius), including Jimmy and Saffi (whom he casually broke up with to chase after a Girl of the Week).
Heloise. In Season 1, she was a Mad ScientistEnfant Terrible rightfully feared by everybody in Miseryville, for her cruelty and sadism surpassed even Lucius. Sure, she had a sweet side when it came to her crush on Jimmy, but she was still an Ax-CrazyYandere who would gleefully hurt Jimmy if he crossed her. Season 2's Heloise is the Only Sane Woman almost solely defined by her crush on Jimmy with barely any moments of Comedic Sociopathy. Worse still, her status as The Dreaded who always had the last laugh was replaced with becoming the biggest Butt-Monkey on the show and being treated as a laughingstock by most characters, while her job as Misery Inc.'s top inventor (and one of Lucius' minions) was completely forgotten a few episodes into the season in favor of making her a generic Gadgeteer Genius.
Lucius was always an example of how The Devil Is a Loser, but it was pretty clear in Season 1 why he was the Big Bad on the show — a megalomaniacal Faux Affably EvilCaligula who reveled in how miserable his subjects were, enjoyed tormenting and mistreating even his own family, and loathed how he could never break Jimmy's positive spirit. Season 2 Lucius is instead an out-of-touch Grumpy Old Man and a strict but well-meaning Only Sane Man who shows no interest in tormenting people and instead wants Miseryville to be a peaceful well-ordered city where everyone loves him for being "cool".
Even ignoring complaints about its quality as a Spiritual Successor to Teen Titans, Teen Titans Go! begins representing its characters very inconsistently during the third season. Raven in particular goes from an introverted, intelligent, snarky girl with a violent streak to being just as stupid, happy-go-lucky, and fun-loving as Beast Boy and Cyborg. This starkly contrasts with the first two seasons in which Raven's intelligence compared to the other Titans was the focus of the episode.
Fauntleroy Fox from The Fox and the Crow is usually portrayed as a suave, sophisticated, well-spoken guy, while being naive and gullible about certain things is generally smart, for whatever reason in the last two Columbia cartoons "Tooth Or Consequences" and "Grape Nutty" he is portrayed as being fat and stupid, speaks in a deeper Simpleton Voice, and acts like a backwoods hick. He is back to being his usual self in the three UPA cartoons though.
This is one of many reasons why Book 2 is considered the worst season of The Legend of Korra, if not the entire franchise, due to derailing its female characters to the point of breaking the franchise's Central Theme of gender equality to the point of Unfortunate Implications. Korra Took a Level in Jerkass. Chief Beifong went from a Jerk with a Heart of Gold to Da Chief. Asami Sato went from a Lady of War that stood up to the only family she has left and curbstomping a gang of ninjas to becoming Chickified to the point of not being in even one hand-to-hand fight, rebounding to her ex-boyfriend out of desperation and sitting back to applaud Bolin for saving her company instead of going down to personally/literally fight to regain her company. Fortunately, Book 3 made up for these slights.
Harley Quinn (2019): Dr. Psycho spent all of season 1 and the first half of season 2 as a Jerk with a Heart of Gold who was nowhere near as bad as his comic book counterpart and had genuine friendships with the crew. Season 2's second half derails him into a sadistic sociopath bent on world domination, turns him from the unscrupulous Anti-Villain he was into a full-fledged villain antagonist, and removes almost all of his redeeming traits. It's like they mixed up the one in the first season and first half of season two with the comic book version.
Star vs. the Forces of Evil: One of the issues some fans had with the fourth and final season of this show was how it subjected several characters to derailment with its most notable victims being the Magical High Commission
Rhombulus seems to have suffered the worst amount of derailment over all the other members of the commission. He was depicted in his first appearance in season 2 as an impulsive but well-meaning Manchild with the mind of a 3-year-old and a heart of gold who formed deep bonds with Lekmet and Star Butterfly. However, season 4 unceremoniously stripped him of all his positive qualities and threw them out the window to instead depict him as a brutal Knight Templar monster-hating bigot who was willing to put lives in danger just to prove a point and rejoice to the act of genocide with a party.
If one stopped to think closely about Rhombulus, they would realize that he is actually 3 characters as the snakes he has for hands are shown to be sentient and have minds of their own. The snake hands were shown to act as voices of reason for Rhombulus in their first appearance but later episodes afterwards treated them both like voiceless sock puppets who would agree with everything Rhombulus did even on doing horrific acts.
Omnitraxus was originally depicted as a mentoring guide and a Reasonable Authority Figure who guided Star into growing up to be a stronger woman. However, he, like the other members of the commission, suddenly became shady and racist bigots and went as far as helping Mina Loveberry ravage Mewni without question.
Queen Moon was seriously derailed in the second half of season 4. Before that, she was shown to be a strict but level-headed and wise queen of Mewni who wanted what she believed was best for her daughter Star and the kingdom of Mewni. However, she was revealed in the episode "Pizza Party" to having orchestrated a coup against Eclipsa by siding with The bigoted and insane Mina and creating a Solarrian army to accomplish this. This happened when Eclipsa already earned Moon’s trust at the end of season 3 and when it was already revealed that Queen Moon’s lineage were not really the rightful rulers of Mewni. Queen Moon did eventually get semi-retailed at the end of the season but her last character arc was all about her attempting damage control after Mina double-crosses her.
Angel's Friends: In season two, Urie suddenly wants to be the chief of the Four-Girl Ensemble like Raf do. Apart from the fact that Ambition Is Evil for an angel, Urie never envied her best friend (or anyone else) in season one.
Zig & Sharko: Marina, who is usually lively and enthusiastic, acts like an insensitive and coldhearted person in the episode "Heart of Stone", for no apparent reason. Sharko spends the entire episode trying to make her feel emotions again.
Lawful Evil (though it lasts only one episode and it's strongly hinted he was actually posing as such for his own advantage; in later episodes, he has no problem breaking laws); later, Ultimate Alien portrayed him as a Manipulative Bastard, which, while being some derailment in a way, was actually good and ended up being an Author's Saving Throw after the Badass Decay he had suffered in Alien Force.