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 is herself derailed from mere Villain Protagonist territory to a sadist in the Brood War expansion, she kills one of Raynor's oldest friends, Fenix, and he vows to kill her because he owes here that much. It lent a significant degree of tragedy to the story, and made his reasons for hating Mengsk even more justified. Come Starcraft II and Raynor kills his best friend with the bullet he'd been saving for Mengsk in order to protect Kerrigan. Or in his own words:
Jim Raynor, SC I: 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.
Jim Raynor, SC II: I never gave up on you, Sarah!
Though he was still planning to kill her at the start of Starcraft2, he just learned that her survival was necessary to the survival of all life in the galaxy.
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.
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.
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.
Fallout 3 does this through Railroading. The game really wants you to make a Heroic Sacrifice and walk into a lethally irradiated room, and while you can be a bastard and have an unambiguously good NPC take your place, none of your followers are willing to do so. Clover, the slave girl mentally conditioned to do absolutely anything her "owner" wants? Refuses. Charon, a ghoul assassin with a similar mentality? Not in his contract. Fawkes, a morally-upright and heroic super mutant who more importantly is immune to the radiation? Claims it's not his "destiny." Happily, the Broken Steel DLC fixes this, and Fawkes even hangs a nice lampshade on the fact that he's perfect for the job.
Despite this, the decision to send anyone other than yourself into the chamber is referred to as cowardly. That's right, you're referred to as a coward if you take the completely logical choice that harms nobody rather than making a pointless and absolutely unnecessary sacrifice.
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-Manipulative 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.
From the same game, Ema Skye. Ema first appeared in the DS version exclusive Case 1-5 and was introduced as an upbeat, cheerful girl with a dark past and an intense love for forensic science. At the end of the case, she left to study forensic science abroad. When she reappears seven years later, she takes over the role of the detective of the game. The girl who was so into science became a police detective instead. In-Universe, it's explained that she did take the test to become part of the forensic science team, but failed. She still retains her love for forensic science, but even that seems to have taken a damper and she isn't as wide-eyed about it as she used to be, and outside of those few moments, she comes across as an extremely bitter young woman who didn't get the job she wanted and decides to just mope about it, doing a half-assed job, instead of trying to change things.
World of Warcraft has not been kind to certain characters from the Warcraft franchise. The need for raid bosses has turned complex Anti Heroes into outright villains, and a distressing number of characters' stories end with "went insane and turned evil." At the same time, the need to justify PvP means that the Reasonable Authority Figures who formed the cross-faction alliance against the Burning Legion in Warcraft III get to be completely marginalized if they're lucky, and if they're unlucky, well, just see what happened to Jaina...
Illidan Stormrage was 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. Someone motivated by both a lust for power and Unrequited Love for Tyrande Whisperwind. In Warcraft III and its expansion he resorted to demonic magic to combat the Scourge, alienating his brother Malfurion, and ending with Illidan forced to serve the Burning Legion, but he's been able to double-cross demonic masters in the past...
But come World of Warcraft and the Burning Crusade expansion, Illidan has undergone complete Motive Decay. He attacked the Naaru in Shattrath City (off-screen) for no reason, became a vicious slave driver to the Broken tribes he allied with, 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.
Surprisingly, Blizzard seems to learn about this and realized it's not too late to undo the damage. So they managed to bring Illidan back for Legion and it seems that he'll be back in the Wild Card status, training a new generation of Demon Hunters to use against the Burning Legion...
Kael'thas Sunstrider was a Well-Intentioned Extremist rather than a Designated Villain, the prince of the Blood Elves who felt that the Alliance had failed his people, leading him to ally with Illidan and travel to Outland in hopes of finding a way to sate their magical addiction. In Burning Crusade, the rest of the Blood Elves are eager to be reunited with their beloved leader...
...only to find that he's now serving the Burning Legion and is capturing Blood Elf pilgrims as slaves. Then in the Magister's Terrace encounter, Kael attempts to summom Kil'jaeden, the most powerful demon lord of the Burning Legion, ranting that he wants to see the Azeroth burn and that he never trusted Illidan to begin with. Explanations for this drastic change range from Kael getting fed up with Illidan's insanity (a Character Derailment chain reaction?) or Fel magic being inherently corrupting, which is bad news for Warlock players. Unlike Illidan, however, Blizzard thought it was too late to fix Kael'thas in the game proper, so the best they can do is to pick his pre-derailed self to appear in Heroes of the Storm
Zul'jin was a hero of the Horde during Warcraft II, portrayed as such in the canceled Lord of the Clans, and even the Darkspear trolls have said "vengeance for Zul'jin!" since Warcraft III. He was painted positively in several Horde quests in World of Warcraft, and the trolls' /charge emote is "For Zul'jin!"
Yet in Burning Crusade he's turned into a hostile raid boss, and not even one treated seriously - the heroes aren't dispatched to stop him by their faction, but by a treasure-hunting redneck. Blizzard tried to make him a Handicapped Badass by cutting off his arm and cutting out his eye - problem is, trolls can regenerate. Although eventually, a few patches later, Zul'jin's position as Raid Boss was replaced by someone else, but he remains vanished and some trolls refer that he's been dead.
Jaina Proudmoore made the difficult decision to abandon Lordaeron as a lost cause and lead her people to Kalimdor, where she was able to ally with an old enemy to stop the Burning Legion. She was even willing to sacrifice her own father to maintain the peace between the Alliance and Horde in Warcraft III's expansion.
And in World of Warcraft, she spent several years doing... nothing. When King Wrynn returned for Wrath of the Lich King, Jaina deferred to his "leadership," even though as the ruler of Theramore and daughter of the ruler of the city-state of Kul Tiras, she was really Varian's peer and arguably the better leader.
And in Mists of Pandaria's conclusion, when the Horde leaders are all gathered for some deliberations, she encourages Varian to up and "dismantle" them, Thrall included, while they're vulnerable. At least by War Crimes she and Thrall have finally made peace. But even then, the novel was treated like it didn't happen, and Jaina spent her time in Warlords of Draenor being obstructive for sending help to the Horde, and by Legion, left Dalaran bitterly when Khadgar attempted to make Dalaran a neutral zone once again.
Related is what happened to Tyrande Whisperwind in Mists of Pandaria. In Warcraft III she's established as the leader of the night elven Sentinels, someone with over ten thousand years of combat experience. Tyrande can still be hot-headed, but tactical about it. But in the quest "A Little Patience" her great idea is to charge a fortified Horde position, leading Varian Wrynn, a forty-year-old warmonger, to suggest a better plan that ends with no Alliance casualties. So like Jaina, Tyrande's character had to take a hit so that Blizzard could try and make the reviled King Wrynn more tolerable. The fact that these derailed characters are both female leads to some Unfortunate Implications. The only damage-mitigation Blizzard could give for both Jaina and Tyrande is to portray them as their pro-peace self and tactical self from their pre-WoW self in Heroes of the Storm, though Tyrande might get something better in Legion.
The legendary daggers questline does this for the Red Dragonflight, whose matriarch is close to a Big Good and acts like a motherly sort of goddess to your character, if occasionally Good Is Not Nice. But in this chain it's revealed they were going to manipulate a newborn dragon, 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.
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 orc chieftain who just wants to be left alone due to shame over his family legacy. Thrall tells him of his father's heroism against Mannoroth, inspiring him to be a leader. But when he returns in Wrath of the Lich King, Garrosh suddenly resents Thrall, challenges his leadership, becomes an aggressive conqueror obsessed with war against the Alliance, and is somehow essentially the second-in-command of the entire Horde. In Catacylsm, Thrall unwisely leaves Garrosh in command, and though still a warmonger Garrosh gets some actual, positive Character Development - even though it's another 180-degree turn from a raging racist and inept tactician, he manages to become a masterful tactician and warchief that, while flawed, is given plot lines that suggest he is growing as a person. But by then the damage was done, and enough players hated Garrosh for Blizzard to give up on him. In Mists of Pandaria he loses any redeeming qualities to become Obviously Evil, and serves as the expansion's Final Boss, and the leading antagonist of Warlords of Draenor for good measure.
Related to Garrosh's example, the entire Horde faction. With the exception of the Forsaken, the Horde was a collection of honorable, shamanistic Proud Warrior Races capable of peaceful coexistence with others - despite being called The Horde they were very much The Alliance in function. But starting with Wrath of the Lich King, a noticeable portion of orc characters started being characterized as bloodthirsty brutes who distrusted their allies, the faction was given the Villain Ball, and half of the racial leaders became Obviously Evil. By Mists of Pandaria all but three orc characters were portrayed as Stupid Evil, xenophobic brutes who slavishly followed Garrosh, and previously positively-portrayed orcs were either Retconned to have always been evil, or were forgotten about. At least when the orcs were evil in the first two Warcraft games they were pragmatic about it and had respect for their allies, with several orc organizations being led by ogres, and trolls being described as their brothers in spirit.
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 alone 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 a 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 extreme viewpoints. This ends up being a rare case where the derailment was justified, as his massive personality shift is a plot point.
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.
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" (which he derisively took to mean "more like humans"), 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 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.
Yukari and Mitsuru go through this in Playable Epilogue for Persona 3. Yukari was fully willing to risk reviving Nyx and cause the end of the world just to see the Protagonist again, and Mitsuru, the Smart Guy, takes her side. This causes the group to become divided and start fighting each other.
The most common criticism of Metroid: Other M is its characterization of Samus Aran, which is nearly impossible to reconcile with the way she usually acts. Samus is traditionally a competent, cunning and fearless bounty hunter who lives to beat the crap out of criminals and confidently hangs around trashy bars wearing almost nothing on her off time. Other M instead presents her as a very submissive and dim woman who is incapable of making her own decisions, terrified of enemies she's effortlessly killed hundreds of in the past, obsessed with maternity, and blindly loyal to a guy who treats her like garbage. If this were a prequel of some sort showing Samus before she became the warrior we know her as, it might make sense, but chronologically this is the seventh game in the series, and in the preceding and following games, she's her normal badass self.
Adam Malkovich gets hit with this as well in Other M. He was introduced in Metroid: Fusion as a very competent, gruff former-CO of Samus', who had a possibly-flirty and certainly friendly working relationship with her before he made some heroic sacrifice to save her life. Also, he isn't dead due to Mind Uploading. Through the game he indeed maintains a professional working relationship with Samus, and he ends up being vital to stopping the unfolding disaster at the end of the game via a superior tactical mind to hers. In Other M? He gets almost his entire squad killed through incredibly poor strategic decisions, treats Samus with at best callous indifference (and at worst outright mentally and physically abuses her), and that heroic sacrifice is entirely pointless.
The Prophet of Truth, when introduced in Halo 2, was shown to be a cunning ruler of the Covenant who was secretly manipulating them for his own purposes. Getting the silky smooth voice of Michael Wincott helped as well. But for whatever reason Wincott chose not to reprise the role in Halo 3, and either because of that or regardless Truth became way less sane in the following game. He was now voiced by Terence Stamp, and his calm conniving manner was replaced with a hoarse yelling dogmatist without any hidden motives. The result that he just ended up sounding like a clone of the Prophet of Regret, Truth's hot-blooded partner (who he betrayed in Halo 2.)
The Didact, the leader of the Forerunners. When introduced in the terminals of Halo 3, he was characterized as a noble idealist fighting the Flood, refusing to fire the Halos that would kill the Flood alongside everything else in the galaxy. But his attempts to find a more humane solution failed, and eventually he was forced to fire to them at the cost of the end of Forerunner civilization. Halo: Cryptum went with this characterization, portraying as a great King in the Mountain, with some unfortunate prejudices but ultimately wiser and better intentioned than the other Forerunner leaders. But then came Halo 4, which turned him into the villain. And not just any villain, but a Darth Vader Clone with glowing skull armor and vampire fangs who wanted to genocide humanity. His objections to using the Halos became out of wanting to keep the Forerunners' supremacy, not to not sacrifice innocents. And then he went even further in Halo: Escalation, where he outright abandoned his principles and tried to use a Halo on humanity. Needless to say, the last book of The Forerunner Saga had some complex explaining to do to reconcile the opposing portrayals, including RetconingHalo 3's Didact into a clone of the original.
Though Halo 4 was a Contested Sequel, one of the most commonly praised elements was its handling of Cortana, your faithful AI buddy for the past ten years, as she reaches the end of her lifespan and struggles to keep helping you while her digital "body" slowly fails. As for Halo 5: Guardians, not only does she turn out to not have died, but she's gone mad with power and wants to enforce Peace Through Superior Firepower on the entire galaxy. The game ends with her launching an attack on Earth, after having killed numerous people on all the colonies she unearthed Guardians on. In short, she's gone from helpful AI companion to an Omnicidal Maniac, all the change in personality happening offscreen, and making all the Chief's attempts to save her from rampancy in Halo 4 feel in vain.
In Ogre Battle: The March of the Black Queen, one ending leads to a character's personality being almost reversed from his behavior in the rest of the game. Normally, Warrior Prince Tristan of Zenobia is a decent guy and Reasonable Authority Figure, willing to work with you, accept you as his vassal, release you from service or even accept ''your' rule or Rauny's over the continent if he thinks that's best. But in the Hanged Man ending, despite the fact that you've won the throne for him, he decides that you are a threat to his rule and that You Have Outlived Your Usefulness, having you murdered to ensure that he has no rivals for his power. This is the only ending in the game where he acts this way - though he'll kill you in the Tower ending too, in that ending it's because you took the throne and became a tyrant.