Bowser in general switches back and forth from a vicious warlord into a simply grumpy and ineffectual villain in the Mario spinoffs with no explanation. A few of the main games, such as Super Mario Sunshine, try to reconcile the radically different portrayals. Shigeru Miyamoto has admitted there is no continuity whatsoever between games, so he can be free to do whatever he wants in each game without being bound by canon. Bowser is whatever the story needs him to be, be it Affably Evil or Not Evil, Just Misunderstood.
Various Sonic characters have been hit with this throughout the years.
Originally, Dr. Ivo "Eggman" Robotnik was a Gadgeteer GeniusMan Child who represented technology in contrast to Sonic representing nature and primarily built his own vehicles and robots to take on the hedgehog. From Sonic Adventure onward, he continually relies on ancient destructive gods which eventually hijack him from villain status. Sonic Unleashed and Sonic Colors however finally allow him to be the main antagonist for once and the latter had him actually be the Big Bad the whole way through.
Tails, originally a Cheerful Child with tech skills, has essentially lost all of his childlike aspects and flaws in favor of being a ridiculously straight faced walking tool kit for the team. His friendship with Sonic has also been diluted somewhat. Sonic Chronicles and Sonic Colors undid some of the damage, though, especially the latter in terms of Tails' friendship with Sonic.
Originally, Knuckles displayed a cunning and intelligent ability to outwit Sonic and Tails; he got tricked by Eggman into thinking that Sonic was the bad guy and wanted to steal the master emerald and was sometimes portrayed as hot headed when it came to protecting said emerald. In Sonic Adventure he was a philosophical Warrior Monk, struggling with his duty as the guardian of the Master Emerald, he was nonetheless calm and cool-headed most of the time and although he got tricked by Eggman again he actually doubted that Sonic would steal a piece of the Master Emerald until he saw Sonic holding a green emerald and thought that it was a shard of the Master Emerald. In Sonic Adventure 2 he displayed a calm and cool-headed ability to get his job done. While he was often fooled and misled, he was still fairly competent. However, as soon as Sonic Advance 2 was released (along with the anime Sonic X) and onward, Knuckles is now portrayed as a village idiot. The Master Emerald is hardly even mentioned and it seems like Knuckles is just hanging around Sonic for the hell of it. In addition to this he also became the patsy of Sonic and friends, as well as often being the butt of jokes at his expense. In Sonic Lost World he only has two lines in the whole game and is portrayed as an incompetent braggart, getting beaten up by Flickies and generally useless. Sonic Battle, Sonic Chronicles and to some extent Sonic and The Black Knight are the only games that return Knuckles to his former glory.
All of Knuckles's motivations revolved around his duty to protect a gem that would doom his homeland if stolen or destroyed. He even spent much of Sonic Adventure reflecting on his life dedication to this task. Later games almost completely omit this, portraying him as merely a laid back adventurer. Even in Sonic Generations, he doesn't recognize the ruins of his homeland. When called to adventure into a dream land in Sonic Shuffle, Knuckles remarked "I have nothing better to do."
Amy Rose. At first, she was a cute Genki Girl with a crush on Sonic, in the Adventure games she is a sympathetic fan girl of Sonic's and even though she sometimes bugs him she shows kindness to others, wanting to help a baby blue bird find it's parents and genuinely wanting to help her friends even being the one to cause Shadow'sHeel-Face Turn. At the end of Sonic Adventure she vows to become stronger and to not merely a damsel in distress so she can impress Sonic with her skills. In Sonic Heroes she is portrayed as a strong leader and an older sister figure to Cream. However, come Sonic Riders, her infatuation with the blue blur almost entirely took over her character. Sonic Chronicles and Sonic Unleashed undid some of the damage.
It got even worse in Free Riders. Ordinarily a kind girl, she becomes an insufferable brat, although this may have more to do with her new voice actress not quite having the hang of voicing Amy yet, making her lines sound brattier than they should be and coming across like a Minnie Mouse clone. She appears to be improving a little in Sonic Lost World were, unlike Knuckles who got the short end of the stick in this game, Amy finally gets to show a softer side like in Sonic Adventure taking care of the tiny animals just like she took care of the Flickie in that game.
Even Sonic himself has suffered from Flanderization issues. In the classic games his "trademark attitude" was mostly limited to: wagging his finger on the game's title screens, tapping his foot impatiently when idling, and being a thrill seeker (riding on planes, snowboarding, outrunning giant bolders). Even with the advent of voice acting in Sonic Adventure he was still pretty much the same, a snarky Jerk with a Heart of Gold who still cared for others. Since Sonic Colors his attitude has now grown to encapsulate his entire character; he constantly makes jokes and sucks the drama out of any scene he appears in. Other characters (except for Tails) aren't allowed to be truly helpful and Sonic isn't allowed to be truly one upped by any of his antagonists anymore (See his interactions with the Deadly Six versus his interactions with Knuckles in Sonic 3 and Shadow in Adventure 2).
Starcraft gives us Jim Raynor and his relationship with Sarah Kerrigan. After Kerrigan is infested by the Zerg and crosses over from mere Villain Protagonist territory at the start of the Zerg campaign to being an outright monster, Raynor vows (after Kerrigan sadistically kills one of their oldest friends, Fenix) to be the one to kill her, because he owes her that much. It lent a significant degree of tragedy to the story, and made his reasons for hating Mengsk even more justified. Then, come Starcraft 2, not only does he kill his best friend with the bullet he'd been saving for Mengsk in order to protect Kerrigan, he outright exclaims, "I never gave up on you, Sarah!" Compare that to what he said right after Fenix's murder:
Jim Raynor: It may not be tomorow, darlin', it may not even happen with an army at my back. But rest assured; I'm the man who's gonna kill you one day. I'll be seeing you.
Honestly, Brood War Kerrigan was itself a derailment. Kerrigan was established in the first game to be an extremely moral person who opposed needless cruelty. She was a killer, but she was still a fundementally good person. Come Brood War she's a heartless sadist who enjoys inflicting pain on anyone for the hell of it.
Mortal Kombat: Shaolin Monks is full of departures from what the characters were like in the second fighting game (which Shaolin Monks is a remake of), from Kitana being under a spell (when she wasn't in the fighting game) to Scorpion randomly attacking the protagonists for no apparent reason. The worst offender, however, has to be Kung Lao; up until then, they made it very clear that he preferred to not be in the spotlight, and purposely kept out of the MK tournament to avoid competing with his friend Liu Kang. In Shaolin Monks, he's turned into a cocky SOB with a constant and heated rivalry with Liu for glory, which became canonical in Armageddon.
Speaking of Mortal Kombat, Kabal easily falls into character derailment. Originally a reformed gangster in Mortal Kombat 3, who joined the good guys after his entire gang was murdered, his ending in the third game stated that he'd turned his lift around. Jump to Mortal Kombat Deception, not only is he restarts the Black Dragon. Although it might possibly be explained that his heel turn was the result of being magically saved from death by a cleric who literally worshipped Chaos.
In Guitar Hero II, Judy Nails was a perky Alt-Rock/Punk girl who was non-ironically described as "always bringing a smile to the stage." For Guitar Hero III, almost every aspect of her was changed to better conform to the violent and aggressive "Punk Grrl" stereotype, including having a permanent pouty scowl and a reference on one of her outfits to being kicked out of Catholic school... not to mention the dramatic wardrobe change itself. And gaining two cup sizes. Her change into a surly punk girl is made even more inexplicable by the fact that "rudeness" was previously listed as one of her "dislikes." She was largely reverted back in Guitar Hero: World Tour. She's been almost totally reverted in Guitar Hero V... but this is accompanied with her character bio being turned into a Take That against everyone who was bothered by the change in the first place.
The same thing happened to the grunge/country/alternative rocker Casey Lynch, retooled into a leather-wearing rocker chick in Guitar Hero III. Her description lampshades this, though, with an anecdote about how she attacked a reporter who accused her of selling out before coldly adding, "I'm sorry, I can afford to pay the medical bill."
Xavier Stone changes dramatically both in personality and physically in Guitar Hero IIandIII; III is especially obvious. Not only is his "cocky virtuoso" personality dropped in favor of "zen master", he appears to have lost all of his copious muscle mass and turned into a No Celebrities Were Harmed version of Jimi Hendrix. It's to the point that it's nearly impossible to believe that he's the same character from previous games.
In Spyro: A Hero's Tail, the Dragon Elders bear no resemblance whatsoever to their previous selves apart from their names, Bentley, formerly a Genius Bruiser, loses the 'genius', and Hunter goes from being dim but good-natured to an irritating Ted Baxter who never gets his comeuppance. And Spyro himself going from a snarky but well-meaning and loyal Kid Hero to an arrogant, disrespectful punk that at least a few players enjoyed being knocked down a peg late in the game.
The Prince in Prince of Persia: The Sands of Time was a somewhat cocky, but likable hero who sets out to reverse his mistakes. In Warrior Within, the in-game story had the Prince as a downright arrogant Jerkass who acted entirely on his selfish intentions. The Two Thrones was originally set to be just as dark as Warrior Within, but after some criticism, the developers actually set up the Prince to confront his actions of the previous games. Much of the game is a conflict between him and the more selfish part of his personality that was dominant during Warrior Within, and he develops to become kinder and more selfless over the course of the game, culminating in a "World of Cardboard" Speech as he accepts responsibility for his actions. even after learning that they caused his father's death.
Mega Man X to some degree. He usually complains about fighting but does it anyway. Then comes X7 and he's a hyper pacifist. Though it's at least understandable given that he wanted to stop fighting in the first place.
Then there's Command Mission, where he's only 1/4 of what his original character was. He has no problems kicking ass and blowing up Mavericks.
Zero displays this. Usually he was just a guy who wanted to complete the mission and more sociable and approachable back in the day, not to mention he was good pals with X, having faith in him being able to change the world for the better (as stated in his ending in X6). Now he's somewhat of a jerk, who doesn't act like he's X's friend at all, although he did try to comfort him at the end of X8. Being fair, it's hard to be open and constantly cheery when your love interest dies (by your hand, no less, even if it was unintentional) and you know that your creation is the catalyst of approximately 95% of the world's problems (in fact, a big part of Zero's personal vendetta against Sigma in X8 is his displeasure in having his data being used to almost cause The End of the World as We Know It).
In the time-management fashion-design game Jojo's Fashion Show, recurring villain Claudio Maximo is a shrewd rival designer whose acts of villainy are fairly subtle and underhanded. The second game shows a bit of character growth on his part by having him become a Friendly Enemy of Jojo who helps mend her relationship with her daughter by gives her a bitchy but effective reality check. The third game, however, has Maximo back in full villain mode with ridiculous evil plots, over-the-top monologuing, and ''Mua-ha-ha-ha''s being spouted every other line.
Dr. Neo Cortex from the Crash Bandicoot series started his transformation from occasionally goofy, yet still seriously threatening mad scientist to a completely negligible goofball who only occasionally even gets in Crash's way at around the third game in the series (where he was revealed to be the Dragon to the real Big Bad of the series), and was extremely deep into this territory by the time Crash Twinsanity rolled around, where he basically turns into a human club, frisbee and snowboard, in that order.
From Tales of Symphonia, Yuan went from the snarky, grumpy, Manipulative Bastard leader of La Résistance, who was almost always a step or two ahead of the party and seemed to have a plan for everything in the original game... to a minor Mister Exposition NPC with almost no personality in the sequel. La Résistance disappeared entirely, although, given that what he was resisting was literally leaving the solar system at escape velocity, perhaps he was just mellowing out.
In Tales of Vesperia, Flynn is unwaveringly Lawful Good to properly contrast with Yuri's vigilante tendencies. He absolutely does not approve of Yuri killing criminals. What does he do in the prequel movie, First Strike? helps Yuri kill a criminal. It's also implied that he, without Yuri's influence, covered up the entire event by falsifying reports. Game-Flynn would have a pretty solid What the Hell, Hero? speech ready upon watching his movie counterpart.
It manages to happen in the same game in Fallout 3. Clover, despite having a slave mentality that makes her do absolutely anything her owner wants will not activate the Purifier in your place if you ask her during the ending, for no obvious reason except that the game is coded to only allow you or Sarah Lyons to make the Heroic Sacrifice here. Charon at least has the excuse that the "contract" his slave mentality is tied to only requires him to perform combat-related duties, though as of Broken Steel he'll begrudgingly activate the purifier.
An even worse example is Fawkes. Despite being portrayed as intelligent and morally upright, despite owing his life and freedom to the Vault Dweller, despite having already performed a similar service in the past, and despite being naturally immune to radiation, he refuses to activate the purifier because it's not his "destiny." This is the same Fawkes who, earlier in the game, single-handedly assaulted Raven Rock to rescue the Vault Dweller. Fortunately this illogical and out of character decision is eliminated once the Broken Steel DLC is added to the game. The final scene is rewritten, and now Fawkes will gladly activate the purifier in the Vault Dweller's stead. He even hangs a little lampshade on the fact that there's no reason for him not to.
It has been pointed that during the final scene, Wesker started showing signs of intoxication, and if that was the case it would make sense that he started making some less than ideal reasoning. However doesn't excuse the entertaining albeit silly Ho Yay that started at the beginning of the game, to the point where he only focuses on somebody else they start shooting at him.
Donkey Kong himself. In most of his games, such as Donkey Kong Country and Donkey Kong 64, as well as the Donkey Konga series, he's portrayed as a pleasant, laid back ape who only gets angry when his enemies steal his banana hoarde. But afteward Nintendo started portraying him as an angry jerkass who beats up animals and steals things for little established reason in Jungle Beat and the Mario vs Donkey Kong series. This has since been overturned by a small change in the plot of the Wii version of the Jungle Beat and Donkey Kong Country Returns.
Happens to Prometheus in the Age of Mythology expansion campaign. It's based on All Myths Are True (well, most of them anyway), and Prometheus is a trickster figure in Greek mythology who is, at least on some level, on our side (see also: fire). So how does he appear in Age of Mythology: The Titans? As a fifty-foot clay monster who seems to have no goals besides smashing stuff and generally ruining people's days, and not only isn't a trickster figure, he doesn't even speak.
One should wonder what getting your liver eaten by a gigantic bird every day does to your sanity.
Ironically in the same expansion, the Titan who's on the side of humanity? She's Gaea, as in the entity (who wasn't even exactly a Titan in the original myths) who caused a huge amount of trouble for humans and the Olympians.
If we're to take the game's campaign as a loose continuation of the Titanomachy, this could oddly go either way, as Gaia was the one who helped Rhea deceive Cronus and subsequently raised Zeus, thus allowing the Olympians to eventually topple the Titans in the first place... but then proceeded to mate with Tartarus, giving birth to Typhon, who she sent after Zeus as retribution for imprisoning said Titans she indirectly defeated. Greek myth is weird like that.
In the original Final Fight games, as well as in his Street Fighter appearances, Guy was a vigilante for justice who was also dedicated to his training of his Bushin-Ryu martial art. Then came Final Fight: Streetwise, and for some reason he became the leader of a Yakuza gang in Metro City's Japantown, and has reduced himself to using guns. There's a reason why Capcom never bothered localizing this game in Japan...
Similarly, Cody starts out being on the side of the city, and fighting to protect people, including Jessica. Then, in Street Fighter Alpha, after becoming dissatisfied with his life, he starts fights for no good reason until he's incarcerated.
In the original Baldur's Gate, Quayle is quite arrogant and stupid. In the series, he's a kindly old gnome who has adopted Aerie, caring for her after her traumatizing experience in the circus that resulted in her losing her wings. This is a rare case of a character having a more positive personality after being derailed.
Quayle explains to Aerie that she changed him for the better, explaining his more compassionate nature. How it made him more intelligent is anyone's guess.
Or for that matter, how either had any influence on the other at all, when he can't have been with the circus for more than a month or two, since he met the PC in baldur's gate, many miles away, a few month's ago, and the PC's travel was probably magically aided.
Also, the novelization derails every single character's personality. Which is why fans don't acknowledge that it exists.
Ironically, the novelization counts as canon within the Forgotten Realms setting, while the game with it's non-linearity doesn't.
However because most of the fans dislike the novelization, Wizards of the Coast declared it uncanon, much to the joy of the others.
A portion of the Final Fantasy fanbase utterly detests the way that Cloud & Co. were characterised in their various spinoffs and cameo appearances. Case in point: Cloud got over his angst in the game, but became Wangstier than ever in both, and Sephiroth underwent Motive Decay to the point where he's just sort of there to torment Cloud and nothing else.
Cloud can be more a case of Pandering to the Base-triggered Character Development in that he is at least given some understandable reasons for his severe depression in Advent Children. Unfortunately, those reasons are only made clear in the longer Advent Children Complete version. Also, complainers about Cloud's supposedly wangsty behavior seem to ignore that Cloud explicitly gets over it by the last third of the movie. This is even more apparent from one of the cutscenes in Dirge of Cerberus — when Vincent talks to Cloud over the phone, Cloud is upbeat enough to snort in obvious amusement at one of Vincent's comments.
Sephiroth in Dissidia borders this. Of all the villains he is one of only two who has no interest in either dominating or annihilating all of existence, and instead fixated on getting his memories back and then endlessly harassing Cloud like a jilted lover. Sephiroth in his own game was a Fallen HeroTragic Villain who wanted to absorb the planet's Lifestream to become a god even if it meant destroying all of existence. Although he does pay lip service to the idea of using Chaos' power to become a god before his final battle with Cloud it seems like he is only doing this to get Cloud to take the battle seriously. He does maintain his trademark stoicism, though.
Phoenix Wright by the time of Apollo Justice: Ace Attorney has completely changed personalities from rather neurotic, hardworking, and perpetually frazzled, to a rather lazy, sloppy pseudo-Magnificent Bastard. Even if you really reach and say that it's a result of what he's been doing the last seven years, there's a sequence where you flash back to him not long after the first trilogy, and he's completely OOC there, most severely in that he's contemptuous of newcomer prosecutor Gavin, telling him that he should have stayed back in Germany and gotten more experience. Considering how he began his own career, and how hurt he seemed when Mia confided that to Maya in the original game, Phoenix would never say that to anyone. If anything, it sounds more like perpetual loser Winston Payne.
World of Warcraft has done this on occasion to recurring characters, but the most notorious examples have been in The Burning Crusade, notably Illidan Stormrage, Kael'thas Sunstrider, and Zul'jin. And now Jaina.
Illidan used to be a sympathetic, if selfish Anti-Hero who tried to do the right thing, but often did so at great expense, using dark magic and hurting his allies in the process. He was motivated by a lust for power and his Unrequited Love of Tyrande Whisperwind. Come The Burning Crusade, Illidan has been hit with the Villain Ball, gone insane like every other major Warcraft villain, and has a complete Motive Decay. Before the events of the expansion, he senselessly attacked the Naaru in Shattrath City for no reason, became a vicious slave driver to the Broken tribes under his control, and convinced himself he actually defeated Arthas during their duel in Icecrown. He then spent all of the expansion sitting atop the Black Temple, doing nothing.
Kael'thas was formerly the prince of the Blood Elves. He genuinely cared about his people and traveled to Outland in hopes of finding a new way to sate their magic addiction. Come The Burning Crusade, when the rest of the Blood Elves travel to Outland, they find that Kael has allied with the Burning Legion and is capturing the Blood Elf pilgrims as slaves. There is very little to explain how the change came about, apart from Kael being fed up with Illidan's insanity and the general knowledge that the Legion corrupts all their followers, but even then, Kael was more firmly entrenched as a Well-Intentioned Extremist rather than a Designated Villain.
Things get worse in Patch 2.4 when he was made a boss in the Magisters Terrace. If people weren't happy about his derailment in Tempest Keep, Magisters Terrace took it Up to Eleven. Kael is helping summon Kil'jaeden - the most powerful remaining demon lord of the Burning Legion - into Azeroth, outright says he wants to see the world burn, and that he never trusted Illidan to begin with. How much is angry conjecture from Kael and how much is the writers rectifying his back story to suit his current characterization is unknown, but the general consensus is that it didn't help what players felt was already an Ass Pull excuse to turn him into a raid boss.
Zul'jin was painted positively in several Horde quests in World of Warcraft, he was depicted as hero in the cancelled Lord of the Clans, the troll charge emote is "For Zul'jin." and the Darkspear have said vengance for Zul'jin since Warcraft III. Yet in the Burning Crusade he's turned into a hostile raid boss. As if that wasn't enough he isn't even treated seriously as an antagonist and his death is portrayed as a joke, utterly ignoring the previous lore on the character.
Jaina Proudmoore, previously someone who would sacrifice her own father to maintain the peace between the Alliance and Horde, turned incompetent somewhere around Wrath of the Lich King, where Varian Wrynn declared war on the Horde, and she did nothing besides ask him not to.
Jaina gets further derailed in Mists of Pandaria, where after a few incidents in Dalaran, she starts to get strangely racist against the Horde. Perhaps understandable given the situation, but very much against how her character was shown prior to this point.
Her newfound racism comes full-circle at the conclusion of the expansion. During the Horde's deliberations to select a new Warchief, she encourages Varian to up and assassinate all of the Horde leaders present, Thrall among them, while they're vulnerable in the wake of Garrosh Hellscream's defeat.
Though if War Crimes is to be believed she and Thrall have finally made peace.
The legendary daggers questline does this for the Red Dragons who previously were Always Lawful Good and who's matriarch was close to a Big Good and acts like a motherly sort of goddess and talks about how she knew when you were born (even if the draenei at least are established as from another planet) when you first meet her even if they or their agents (i.e. you) do what they have to do to maintain peace. But in this chain? They're initially established as Good Is Not Nice but then its revealed they were going to manipulate a new born dragon baby, had experimented on him, planned to control his life, and planned this all in front of him. While the last is a case of Too Dumb to Live its a far cry from their previous characterization. Or concurrent since their matriarch is concurrently acting as something of a Big Good and even brings Brought down to normal on herself to help defeat the Big Bad. Depending on the writer?
Garrosh Hellscream is less an example of Character Derailment as much as he is an example of outright character inconsistency. In The Burning Crusade, he's a depressed chieftain who just wants to be left alone due to shame over his legacy, which was saved by Thrall's telling of his father's heroism against Manneroth, inspiring him to be a leader. Next expansion, he suddenly resents Thrall and challenges him to a duel to the death, becomes an aggressive conqueror obsessed with harming the Alliance, and is essentially the second-in-command of the entire Horde. Then, from Wrath to Cataclysm, Garrosh actually gets some positive Character Development, though again 180-ing him from a raging racist and inept tactician to a masterful tactician and warchief that, while flawed, is given plot lines that suggest he is growing as a person. Then finally, is then derailed utterly and completely in Mists of Pandaria, where he becomes Obviously Evil without any redeeming qualities at all.
The entire Horde faction. With the exception of the Forsaken, the Horde was seen as an honorable, shamanistic faction of potentially aggressive, but still well-meaning, races banded together. Over time, the entire faction has been given the Villain Ball, now with half of the races with Obviously Evil racial leaders and many of the formerly good races, like orcs, being treated far more negatively. This can get especially confusing when it comes to the orcs, as they went from good to evil with no real explanation offered within the game, beyond a leadership change. They started out as harsh Proud Warrior Race, yet also accepting of their allies treating them as they would other orcs, as well as being composed of a diverse set of individuals(they were the Horde counterpart to humans for a reason). Orcs then had a code of honor, some even being Warrior poets, and most tried to maintain their peace treaties, even when they didn't agree with them. Starting from WOTLK, a noticable portion of Orc characters started being characterized as Bloodthirsty brutes who distrusted their allies. By the time of Mists Of Pandaria, every orc barring just Three Characters were portrayed as Stupid Evil xenophobic brutes all of whom slavishly followed Garrosh as if they were a Hivemind. Previously positive orc characters were either retconned to have always been evil, or forgotten about. Even when the orcs were evil as in the old Warcraft games, they were still pragmatic, and had respect for their allies, with several orc organizations being led by ogres, and trolls being described as their brothers in spirit. Basically Extreme Character Derailment on an Entire race!
In Super Robot Wars K, Major Zairin suffers from this hard. In his origin series (Zoids: Genesis) he's the Worthy OpponentRival to Kid Hero Ruuji, but that just means he considers him a good pilot and enjoys fighting him (And will do if possible). In the game he FOLLOWS Ruuji and seeks for rematches when possible, going as far to ally himself with Proist and Gil Barg to get to face Ruuji. Thing is, Proist and Gil are villains while Zairin's a pretty honorable guy, who in canon (And also the game) does a Heel-Face Turn after finding out his boss Emperor Gene is also a villain, so Zairin siding with the two jerks makes no sense. Way to go, Banpresto.
Anders in Dragon Age: Origins – Awakening is a Heroic NeutralDeadpan Snarker mage who wants to be left along from the Templars to have his own harem. At one point in the game, he even comments on how much it is a bad idea for the entirety of the mages to rebel against the Templars. Anders in Dragon Age II is melancholy Knight Templar who takes it upon himself to be the defender of mage rights, and is completely dismissive/hostile towards those who disagree with his extremer viewpoints. This ends up being a rare case where the derailment was justified, as his massive personality shift is a plot point.
Plus you can see him slowly changing over the course of the game. When you first encounter him in Dragon Age II he's notably sullen and not quite as cheerful as he was in Awakening, but he still cracks a joke and smiles once in awhile. He gets along with just about everyone, but is occasionally irrating about his mage justice. As the Acts of the game (3 of them, between which about 3 years pass in the story) he gets worse and worse, until he's always quiet and sullen and everyone except Varic (and possibly Hawke) is sick and tired of him and his crusade all the time and only put up with him because he hangs out with Hawke.
It should also be noted that Anders is only irritating if you don't agree with him. If you agree with him that the Circle system is unjust and Mages are being mistreated, then Anders' constant anger about it makes perfect sense: he's frustrated that everyone is willing to condone (through their silence) unjust domination and dismiss the guy trying to bring it to their attention, without ever giving the guy a fair and serious hearing. As a real-life parallel, Martin Luther King Jr. would often become extremely annoyed and agitated, but that was because most whites were simply not willing to see how much they constantly made black people suffer when, to our modern ears, it's perfectly obvious. So, whether or not Anders has been derailed depends not on the mages vs. templars conflict but depends on where you stand when it comes to what does, and does not, constitute ethical and unethical social systems. If you think it's unjust to imprison people because of what they might do, then Anders is perfectly justified in being angry at the citizens' collusion with an evil social order; as Desmond Tutu said, if you remain neutral when an elephant steps on a mouse's tail, the mouse will not thank you for your neutrality. If you don't think it's unjust, then Anders becomes insufferable to listen to, because the meaning of both Anders' behavior and the behavior of the rest of the characters changes depending on whether or not you think sometimes it is possible for an idea to both be "the way things are according to most people in a society" and completely unethical. Either Anders is an annoying prick and the rest of the citizens just want to be left alone, or Anders is a Socrates-level gadfly who's trying to show the citizens how awful they're being on a daily basis.
Played for either laughs or serious tones in BlazBlue for Jin Kisaragi, depending on where it is. Jin is normally level-headed and The Stoic to the cast, never fretting over anything... until Ragna appears. This is his cue to be insane, with his speech becoming littered with... overtones. However, in his Gag Reel and the bonus material and Fourth Wall Mail Slot "Teach Me, Miss Litchi"/"Help Me, Professer Kokonoe", the overtones are lampshaded and Up to Eleven. Nobody has yet to complain, possibly because fans are only using the jokes that Arc System Works started. It should be noted that this change is only in bonus material, and does not affect the main story.
Continuum Shift showed Nu-13 being turned from a cyborg that's either very machine-like or a Yandere-Fangirl to a cheerleader-stereotype. However, much like Jin's personality in Gag Reels and bonus selections, Nu-13 only appears in the Gag Reels, and much like Jin, this is played for laughs.
Krystal changed from a decent-looking, somewhat useful and not quite helpless addition to the team into an Ax-Crazy jilted-lover in Star Fox Command.
Many argue League of Legends did this to Viktor with the release of Jayce. Viktor is a scientist whose motivation was bitterness over his inventions being stolen and a desire to prove his own skill. In Jayce's lore, Viktor steals Jayce's invention. To add further indignity to this shift into blatant hypocrisy Jayce then creates a new device and singlehandedly storms Viktor's lab, defeating him and his acolytes on his own turf with a weapon he created on the fly. Even by League of Legends standards the forum response has been fairly venomous.
As soon as was possible without being an admission of utter failure the lore was modified so that Jayce comes into possession of a unique crystal instead of his own invention, which Viktor seizes after his offer of alliance was rejected, and Jayce manages to destroy the crystal instead of defeating Viktor outright.
Gilgamesh in Fate/stay night is an arrogant jerkass with a god complex. While he was like that at one point in the mythological canon, he was humbled and had character development into a better person after he failed to obtain the secret of immortality. Justified in a Doylist sense even if not a Watsonian one, in that he was needed to be a villain and his having been humbled already would have ruined that.
Those who romanced Jacob Taylor in Mass Effect 2 will discover that his love isn't as genuine in Mass Effect 3 when he not only cheats on Shepard while she's impounded but also manages to knock up another girl at the same time. Keep in mind that Shepard was only in jail for six months and it's implied that the relationship has been going on for a while. He lessens his derailment somewhat by at least admitting he's been a bastard when FemShep calls him out on it. It doesn't help that his writer transferred to the Dragon Age team after ME2 was completed.
How about Cerberus' top hitman Kai Leng, Who when introduced in Retribution, comes off as ruthless, intelligent and quite the manipulator. Come his appearance in Mass Effect 3 Leng is stripped of any of his more engaging traits and is little more then his boss' named Elite Mook.
Liara T'Soni is a lesser example. In the first game, she's an innocent archeologist and rather naive about the various dangers present within the galaxy. By the second game, she has become a ruthless information broker and killing people in her way isn't an issue in her war against the elusive Shadow Broker.
It's worth noting however that these differing portrayals are later reconciled in the Lair of the Shadow Broker DLC, where she admits that this was simply a front put on to survive in the criminal underworld, a coping mechanism to cope with Shepard's death, in addition to the lingering guilt over turning Shepard's body over to Cerberus, even if it was to prevent the Shadow Broker from acquiring it for the Collectors.
The Geth in Mass Effect 3 suffer from this by deciding to upgrade themselves to true AI status by using Reaper code. So much for their philosophy of self-determination and not cheating by skipping ahead.
Somewhat understandable as the main writer for the AI characters in the second game, Chris L'Etoile, did not return for the third game and mentionedin an interview to have been unhappy with all the changes forced upon him to make the AI characters more "emotional", despite being creatures supposedly of pure logic.
Lara Croft in the Tomb Raider franchise goes from being witty with a sense of danger and thrills to throwing herself into harm's way at every chance she can get just to get some artifacts that contain power (she only studies them and doesn't actually use them for personal gain) while killing any person that gets in her way. By the Angel Of Darkness installment, she becomes a near total Jerkass to everyone around her. The Crystal Dynamics reboot attempts to throttle back the derailment of Lara's character and make her seek out dangers for thrills and throwing snark and wit at anyone who deserves it while also becoming compassionate when the scene calls for it.
Athena in God of War series: In the two first games and spin-off she's etablished as a benevolent deity and the only sane woman in the pantheon. While she can't go against Zeus' will, she shows compassion for what happens to Kratos and remorse for what happened to his brother by her fault. At the end of the second game, she perfoms an Heroic Sacrifice to save Zeus (because killing Zeus would lead to the destruction of Olympus). Comes the last game of the trilogy, she comes back as a ghost and encourages Kratos to kill Zeus because she understood his death is necessery to free humanity. And then at the end of the game it's revealed she was manipulating him in order her to become the new Queen of Olympus. When Kratos decides to give the power of Hope to humanity instead to her by sacrifice, she left him to die angrily.