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 tightass 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.
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, and Skarloey, previously one of the oldest and wisest engines on the island, is now childish and afraid of everything (from thunderstorms to the incline yards to the wharf). Thomas himself was once depicted as friendly and helpful, if somewhat cocky and brash. He's now a poorly-written Creator's Pet that is obligated to make appearances in every single episode (even episodes about the narrow gauge engines) and dispense advice to everyone.
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 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.
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 Not Bad, 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 ocasionally 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 in the start of the series, he is seen as a sensitive if somewhat snarky pushover but transforms to 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.
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 episode, 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 Simpsons is a major offender: Homer going from a well-intentioned buffoon who was fundamentally a good person under his many flaws to to a hateful Jerkass who usually has little empathy for those around him, Marge going from a sensible, down-to-earth, and nurturing woman who was deeply supportive of her kids to a ditzy, snappy, and hypocritical housewife who has openly stated she would be much happier without her children, Bart going from an underachieving Brilliant, but Lazy prankster and troublemaker to a full-time sociopathic delinquent who can barely count to 10, and Lisa going from an idealistic Child Prodigy who was still just a little girl at heart to an ultra-liberal, activism-obsessed, egocentric, dramatic mini-teenager with little sympathy for those who go against her beliefs. And then there's Maggie, who's mostly a cute baby with a pacifier, but starting with "Who Shot Mr. Burns?", she exhibits a darker side in certain episodes (however, many fans consider that one of the better cases of this trope, since MaggieTook a Level in Badass).
Ned Flanders, when he first appeared, was a perfectly nice guy. He was religious and more than a little repressed, but he wasn't hurting anyone and he seemed to love all people unconditionally. In fact, the whole joke was that Homer hated him for no reason. He was the kind of neighbor most people would be lucky to have, friendly, kind and always willing to lend you whatever you needed. Then came his Flanderization, where he went from a generally nice guy to his ludicrously meek, inoffensive, and Christianity-obsessed persona. The derailment kicked in later, when the Simpsons writers decided they wanted a Right-Wing Christian Strawman to mock and so they derailed Flanders to make him fit the bill.
And maybe this is more Early Installment Weirdness than character derailment, but in Flanders' very first appearance he was so thoroughly insensitive to Homer's obvious financial troubles that he came across as a complete Jerkass fully deserving of Homer's enmity (though Ned may not have been aware of just how bad Homer's financial problems were).
In fact, some recent episodes show that the derailment has effectively inverted the Ned-Homer dynamic, as now Homer is a happy-go-lucky idiot with Ned loathing him effectively just for existing. One episode actually covers this, in that Homer realized the reason he hated Ned was because he always found him better than him and moved on, while Ned's reasons to resent Homer in later seasons is that Homer keeps calling him Stupid Flanders within earshot of him and demeans him in front of Rod and Todd, they do end up making amends, though.
C. Montgomery Burns is a big victim of Villain Decay. In early seasons, Burns was a downright sadistic and evil old miser who generally treated his employees like crap, stole from children, tried to block out the sun (for money), violently mutilated sea animals to make Lil' Lisa Slurry and planned on turning a litter of puppies into a fur coat. Turn to later seasons and he's just a feeble old man who is more ineffectual than anything else and serves more as fodder for jokes about being behind the times then anything else. Although every now and then he gets a Forgot Flanders Could Do That moment and ends up doing plot such as launching his environmental hazards on the Amazonian forest or putting Mona Simpson on the run from justice out of spite.
Patty and Selma have shown the most extreme case of derailment than anybody else. They were originally just a couple of gruff curmudgeons who hated Homer and think Marge should divorce Homer, take the kids with her, and remarry. Later seasons would add appalling housekeeping and near-criminal incompetence at their jobs (both arguably even worse than Homer's), not to mention smoking everywhere, including around children and on the job at the DMV (which nearly gets them fired — until Homer takes the fall for the both of them). Selma would go through three astonishingly bad marriages (and she only wised up after realizing how long her last name was), while Patty became a full-fledged militant lesbian (despite that "Principal Charming" established that Patty is heterosexual and willingly celibate, while Selma has had celibacy thrust upon her and wants to have a husband and kids before she dies alone and unloved). Of course, all this pales next to becoming a new contender for Most Violent Simpsons Characters, being utterly obsessed with trying to end Homer's marriage, including celebrating his death by buying a tombstone for Marge, trying to kill him by pushing him off a bridge (though that was part of Homer's plot to land on the boat where his "Congratulations on Completing Community Service" party is being held, the fact that Patty and Selma were willing to kill Homer just because is unsettling) and attempting again later by reenacting the famous bathroom torture scene from the first Saw movie.
Moe is also worth a mention. When he first appeared, he was merely a surly and cynical bartender, who was a pretty decent guy despite his rough edges. In later episodes, he's a miserable wretch and registered sex offender who conducts numerous illegal operations, tries to kill himself every other episode, and constantly lusts after women in revolting manners.
Otto counts too. The character was originally depicted as a laidback youth who got along great with kids and was generally a really fun guy to be around, despite his humble status as a bus driver, his carefree nature, and the occasional hint that he was a pot smoker. Later on, he devolved into this wretched and dangerous lowlife with an insane addiction problem to any illegal and hazardous drug he can get his hands on — a trait gets brought up every time he appears now and almost entirely defines his character in post-Golden Age seasons.
Stewie, previously a humorously sociopathic Enfant Terrible with matricidal tendencies, has become Brian's effeminate sidekick (the Doylist explanation was that the writers felt that they'd run the evil baby joke into the ground) though his violent tendencies do occasionally show up.
Brian has changed from a snarky intellectual ironically portrayed as vastly more intelligent than his owner Peter, to mainly a pretentious and egotistical douchebag, a preacher for the writers' liberal political views and The Chew Toy concerning his inability to quit drinking, be a good writer, and hold a relationship.
Lois, while at first as the down-to-earth one of the family, albeit with a somewhat healthy sexual appetite, has become an insane nymphomaniac and Abusive Parent. Being married to and living with a man like Peter for years could easily drive the most sane, sensible person crazy. One episode even implies this by showing a tumor in her brain after she mentions bottling up all the stress that Peter causes her.
Meg has evolved from merely a unpopular high-school girl trying to fit in to an abused, suicidal psycho of epic proportions.
Peter himself, while always a bumbling idiot but with slightly good intentions (like Homer Simpson, only fatter, brasher, and more into pop culture from the 1970s and 1980s), has become a JerkassPsychopathic Manchild (making Homer Simpson look like the clueless, yet caring Bumbling Dad he was in the early days of The Simpsons).
Originally, Quagmire's obsession with women and sex didn't go farther than having a 50s swinger-type vibe, and was for the most part a happy-go-lucky Casanova Wannabe. He quickly became more successful in his pursuit of women, and eventually turned into an out-and-out rapist with a few offhand references to some of his conquests being underage girls. He went through a second derailment in later episodes, with his intense (and randomly produced) hatred for Brian and Self-Deprecation based neuroses becoming the focus of a lot of his humor. He eventually became far more bitter and cynical even in scenes without Brian, as opposed to the more carefree, fun-loving person he used to be.
Joe became overly macho to compensate for his handicap as opposed to his original character which was more like a good cop who happened to be crippled.
There's something to be said for the character derailment of Cleveland Jr. for the run of The Cleveland Show how he used to be a hyperactive child to a fat and slow teenager (though considering that Cleveland, Jr. disappeared after he kicked around a golf ball like a soccer ball in "Fore Father", who knows what could have happened to him between "Fore Father" and his appearance on The Cleveland Show's pilot episode?). The Cleveland Show episode "A Rodent Like This" reveals that the fat, weird, slow Cleveland Jr. is an impostor who killed the original and took his place and nobody noticed because they were too stupid though that may have been a simulation. Like most of the Griffin family he also Took a Level in Jerkass during that show's final two and a half 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 to 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 season 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.
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's 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.
Numerous examples in the Total Drama series, especially during Total Drama Action, Total Drama World Tour, and Total Drama All-Stars.
Trent. In Total Drama Island, he was "the Cool Guy" and the closest thing to a normal teenager: friendly, easygoing, grounded, humble, and talented. He developed a thing for Gwen and they end up going out on the last episode. By the fifth episode of TDA, his relationship with Gwen has turned him into a clingy, neurotic, and desperate loser who has to please her since he's so worried about her feelings dwindling since she has more in common with Duncan. It gets so bad that he starts throwing challenges for her and Gwen has to convince his team to boot him off. And that's not even getting into his sudden maniacal Super OCD obsession with the number "9".
Geoff and Bridgette both completely lost their personalities in TDA and TDWT. Their relationship transformed from ordinary teenage lovers with a supportive emotional connection to each other and many major friendships with other contestants to being excessively physical and hormonal Makeout Kids who spend the other half of their time bickering with each other and were basically Satellite Characters to each other (little wonder the popularity of the "Gidgette" ship massively declined after TDI). Geoff then Took a Level in Jerkass on the Aftermath segments and turned from a laidback, fun-loving Nice Guy into a sadistic, narcissistic Small Name, Big Ego who loves torturing his fellow contestants, obssesses over his appearance, only cares about fame and money, and generally keeps failing Bridgette as a boyfriend. Meanwhile, Bridgette was derailed from a compassionate, sensible, easygoing, and tomboyish Granola Girl to a shallow, bitchy, hypocritical, and nymphomaniacal Clingy Jealous Girl who violently beats up Geoff for looking at other girls or insinuating she's fat, breaks down when she realizes her boyfriend won't be in TDWT with her, becomes completely smitten by Alejandro and tries to kiss him on national TV, and generally cannot survive for 5 minutes without satisfying her physical and emotional needs for big, strong, handsome men.
In TDA, Courtney went from an uptight and bossy but well-mannered and well-meaning Good Counterpart to Heather who had a sense of honor in playing the game to a violent, spoiled, manipulative, and cutthroat Jerkass who sues if she doesn't get what she wants (something she only did once in TDI, in a justifiable case) and cares only about what she wants, becoming pretty much as bad - or worse - than Heather was. And then things got worse after Duncan cheated on her with Gwen in TDWT (done solely to kill DuncanXCourtney and make then-Fan-Preferred Couple DuncanXGwen canon). Being furious and upset over the incident at hand is one thing (and completely understandable), but actually losing challenges on purpose for no reason other than to spite Gwen is worse.
Things just get worse for Courtney in TDAS. At first it appeared as if the writers were finally trying to get Courtney's character back on track. After the whole Duncan incident, Courtney finally became Gwen's friend again and the two were just as close as ever, and she even pursued a new romance with Scott. It looked as though Courtney finally had a chance to be likable again. All that development was suddenly derailed in one entire episode. Out of nowhere, Courtney reveals that she has already planned out what she's going to do now that she's in the final five and what order she plans on getting rid of the other contestants, two of whom are her close friend and new love interest. She chooses to backstab them because a million dollars is worth more than her relationships with either of them apparently.
Happens to Gwen in TDWT, where she goes from a moody but loyal and kind-natured Misunderstood Loner with a Heart of Gold to being an selfish, unrepentant, and manipulative jerkass. She does an example of Moral Dissonance by kissing Duncan while he was still with Courtney. She feels some remorse for doing it at first but when the kiss is found out, she doesn't try to vote herself out like the last season or apologize. And later she manipulates Cody's feelings for her to vote Courtney off. She's even lampshaded as being "New Heather".
It only got worse for Gwen in TDAS, with many seeing her as having become "too girly" and too concerned with what other people thought of her (such as giddily gushing with Courtney about each other's hair and clothes), as well as her handling of her second fallout with Courtney, which made her out into a hypocrite for treating Courtney the exact same way she had been treating her in TDWT. It should be no surprise at this point that she has gone from one of the show's most universally loved characters to one of its most divisive.
Leshawna, most definitely. TDI had her as a girl who would take crap from nobody and could get quite feisty, but was still very kind, loyal, caring, and friendly, to the point where her "elimination" came in part because everybody who had been eliminated at that point liked her so much. Then comes TDA, where, for absolutely no good reason she manipulates her teammates into giving her a reward by pretending to cry and being sentimental, only to trash-talk them behind their backs. It's even worse in TDWT, in which she's now a massively arrogant jerkass and a gargantuan hypocrite with little to no redeeming qualities, and goes right back to hating Heather even after supposedly making up with her in TDA; even beating Heather up when Heather is trying to warn her about Alejandro's villainy and, after learning that she was right, is still proud of doing it. Basically, much like Courtney, she went from being Heather's moral and ethical superior to being just as bad as her in every way (in fact worse, due to Heather's Character Development around the same time to a minor Jerk with a Heart of Gold).
DJ too. He was introduced as a sensitive, kindhearted, and easygoing Lovable Jock who was capable of being brave, tough, and even mischievous or playful when the time called for it (like overcoming his fear of water to help his team win a challenge, standing up to Chef for forcing him into an illegal alliance, and joking around with Geoff and Duncan). But in TDWT, he's become a gigantic crybaby who Screams Like a Little Girl at virtually everything, constantly cries and blubbers for his mother (something he never did once in TDI), and is completely useless in challenges due to being such a cowardly wimp that he can't concentrate on anything except his own fearfulness.
Duncan also qualifies because during his return in TDWT, he went from a carefree Jerk with a Heart of Gold who actually did have some degree of moral standards to a remorseless asshole with no redeeming qualities who would stoop to any level to be bad. Throughout the season, he holds the Conflict Ball, cheats on Courtney with Gwen, never helps his team, cheats during the game, treats almost everyone like crap (especially Courtney), becomes a Manipulative Bastard, and helps Alejandro stir the pot for no good reason. It's no wonder he went from being one of the show's most beloved characters to one of its most controversial as a result of the season.
Owen also counts. He started out as an extremely friendly big lovable oaf who was willing to try anything in the name of fun, up to and including dating the Ax-Crazy Izzy. Suddenly in TDA, he becomes a self-centered Small Name, Big Ego and an obnoxiously moronic Jerkass who can't stop himself from eating for one second and doesn't care in the slightest when his actions harm others, while in TDWT, he decides out of nowhere that he wants to break up with Izzy for being crazy despite having been totally infatuated with her insanity for the previous two seasons.
Lindsay was set to to grow leadership skills and develop some brainpower in TDA, but that was suddenly thrown out of the window with her elimination (in which she voted herself off by accident) and never revisited afterwards. And then she ends up getting dumber and dumber with every season, voting for herself near-constantly in TDWT and being unable to understand simple concepts like pushing and pulling in TDAS.
Like all of his team that season, Harold was hit by this in TDWT. In the previous 2 seasons, he was a well-meaning and intelligent Crouching Moron, Hidden Badass whose "mad skills" rescued his team on more than one occasion, thus proving himself a good teammate whose boasts were almost never empty. Suddenly, in TDWT, he's become an unbelievably arrogant Small Name, Big Ego and Know-Nothing Know-It-All who does nothing worthwhile to help his team in challenges (unless constantly prattling on about himself and his "knowledge" is considered something) and whose only contribution is being hit with the Idiot Ball and getting convinced by Alejandro to perform a Stupid Sacrifice.
From the cast of Revenge of the Island, we have Zoey and Cameron, thanks to their friend (well, boyfriend in Zoey's case) Mike and his Superpowered Evil Side Mal. Zoey went from a grounded but somewhat gullible girl who was able to stand up to herself when needed to being completely helpless to the point of needing Mike to help her win challenges (despite keeping the skills and abilities she had developed) and being Too Dumb to Live by completely unable to recognize that Mike was actually Mal despite being warned by three different people that Mike was Not Himself (to the point where she actually thought Alejandro was warning her about Courtney in one episode) and being aware of his DID from the previous season. Cameron, despite being established in his debut as one of the show's most intelligent characters bordering on Impossible Genius, stupidly trusted Mike when he was Mal, somehow failing to recognize some of the very obvious signs something was wrong with his friend even though he was one of the first to discover Mike's mental disorder in the previous season.
Dave from the cast of Pahkitew Island is a very extreme case as he was derailed in the very season he debuted in. At the beginning of the season, he was established as a pessimistic and neurotic Deadpan SnarkerOnly Sane Man who also happened to have a crush on the friendly and grounded Passionate Sports Girl Sky. But around the 6th or 7th episode he's completely lost all his other traits to focus solely on his crush on Sky, as he started becoming too dependent on her which led to him suffering from an emotional breakdown when she and Max switched teams and ultimately voting himself off in the 9th episode when she rejected him (albeit rudely due to being in a bad mood at the time), leaving him a bawling wreck and beginning a Sanity Slippage. Things got even worse in the finale when he completely loses his mind after learning that the reason why Sky rejected him was because she already had a boyfriend back home (who she tried to break up with prior to joining the show), conducts a huge FaceHeel Turn, and abruptly becomes the Final Boss of the season in order to thwart her chances of winning the million (and actually succeeds in one ending).
Cosmo in The Fairly Oddparents got 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.
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. 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).
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.
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 Season 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).
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.
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.
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 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 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 he 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 to Eleven.
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 meganekko in the first season. In season two, she became rather abrasive to her schoolmates and friends, showing occasional signs of emotion.
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 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 of 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 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.
Harley Quinn (2019): Dr. Psycho spent all of season 1 and the first half of season 2 as a Jerk with a Heart of GoldGrumpy Bear 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 on 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 Moons trust at the end of season 3 and when it was already revealed that Queen Moons 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.