Priss Asagiri falls victim to this in the final episodes of Bubblegum Crisis Tokyo 2040. She reveals to her love interest, ADP Officer Leon McNichol, that the real reason she hated the AD Police was that they broke her favorite CD during a boomer attack. Before then, she was developed as a complex, mysterious, stoic loner. This revelation derails her integrity for the sake of cheap laughs and misguided social commentary.
The fact that Leon becomes her love interest at all was the start of Priss' derailment. She'd previously treated him as not even worth her contempt, but he barges into an empty club with her one night and applauds (in what's actually a very condescending and smarmy way) for her while she's standing onstage, and all of a sudden they're dating. And all of a sudden she stops paying any real attention to Linna. There's a reason that it's easy to consider everything after the first seven episodes Fan Discontinuity.
Daichi Misawa of Yu-Gi-Oh! GX, whose Teen Genius personality became grotesquely twisted out of shape until he began acting directly contrary to what had previously been shown. (Then again, a drastic personality change is Justified if he really was raped by a love-crazed Amazon.) He eventually snaps/has an epiphany and runs naked through the school grounds a la Archimedes, and then gets Put on a Bus. He eventually returns but does little before deciding to live in another dimension with said love-crazed Amazon. This was arguably a minor attempt to return his character to its roots as he made this decision partly because he felt more appreciated in the other dimension than he did in his native one and acceptance/recognition was what he had sought from the start.
Yu-Gi-Oh! 5D's: For quite a few people, the change in Aki's character was too abrupt, most of it within the space of one or two episodes rather than being more evenly spread out. To clarify: The only thing the former Aki and the recent Aki have in common are their physical attributes.
In Transformers Generation 1, Cyclonus and Scourge were a pair of dangerous, cunning, devious warriors. In Transformers Headmasters, they suddenly became a pair of bumbling idiots who kept getting in Sixshot's way, and wondered why Soundblaster lost all respect for them. They were quietly dropped from the show soon after.
Rodimus Prime after the movie also suffered derailment, turning from 'plucky youth' into 'useless jerk' over the third and fourth seasons. It's like they completely screwed up the chance to show a less invulnerable, more three-dimensional Prime.
Nina Wang got hit with this hard after Episode 16 of Mai-Otome. Before, she was a somewhat friendly rival to Arika and a more-than-competent fighter on track to become one of the greatest Otome ever. Afterward, she underwent a Face Heel Turn and killed tens of thousands of people to prove her loyalty to/love for her "father", and her rivalry was thrown on the back burner for all but one or two episodes during that period, and her position was snapped up by Tomoe for reasons unknown to Arika.
This occurred in Tytania, and it happened to, of all people, Estrades- the man turns from a calm, devious schemer who was perfectly fine being ordered about by his brother Ajman to a wild, rabid, hateful person who despised his sibling with every fiber of his being. To top it off, his derailment is combined with a severe case of a mixture of the Idiot Ball and Too Dumb to Live when he offs himself by shooting himself after tumbling down a flight of stairs.
Dumas from Kiba starts off as the strong, quiet champion of Tempura/Templer, then makes a shocking Face Heel Turn and would have lead Jimoto's invaders to overtake Tempura had not Zed's Spirit gone on a rampage and stopped them. The really interesting part was that Duma's reasons for this were pretty reasonable: because of his family history, he basically wanted to shape his own destiny. However, after those events, he became a regular psychotic baddie whose sole purpose in life was betraying Tempura in ever way possible and becoming royalty.
Brock from Pokémon was the voice of reason with occasional attractions towards girls in Kanto. When he got back from his bus trip during the Orange Islands Filler Arc, he became a super freak with raging hard-ons every time he sees things with two legs and a vagina. The writers caught on by the end of the Sinnoh arc and gave him a mercy bus ride.
And his voice of reason status has apparently been reduced to just explaining to his comrades (and the audience) every single move that gets made during a battle.
Considering that Ash has battled various opponents in the past with impressive feats, you'd think he'd be able to beat Paul or Trip once without getting his ass handed to him. No matter what, post-Sinnoh Ash seems to fight like an inept beginner, especially considering his impressive victories in the past; and don't get anyone started on the ass whipping he gave to Fantina in a rematch.
It was explained a few appearances in that Paul is pretty much just as experienced as Ash is. And while that is admittedly an Ass Pull, there is still no question he is smarter and a better tactician in the heat of the moment compared to Ash. Trip on the other hand, there is no excuse whatsoever that Ash should lose, or even have trouble with a beginner trainer, at all.
On that note, Paul's transition from Jerkass to a more considerate and caring trainer at the League tournament was way too abrupt and made little sense, with little to no explanation to it other than being used to finally put an end to his rivalry with Ash.
In mid-Sinnoh, Ambipom was literally Put on a Bus, for incredibly contrived reasons that made no sense given the Pokemon's previously established character. From the beginning Aipom/Ambipom was shown to love Contests, which eventually led to it getting traded to Dawn, the Contest Coordinator of the group. DP 124 has the characters participating in Pokemon Ping Pong, and Ambipom suddenly decides to leave her trainer and the Contests she loved so much to go off with a complete stranger and train for an activity she didn't even know existed before the previous episode. Many fans are not happy.
What in God's name happened to Graham Aker in Season 2 of Gundam 00? In Season 1 he was an obsessive Flag pilot, and totally loyal to The Union. Then in Season 2 he's...an obsessed Japanese Otaku calling himself Mr. Bushido? Who acts like he just walked out of a Super Robot show? What. The. Hell. And the fanbase that decries the situation in Seed/Seed Destiny says nothing about it? Yes Graham's transformation is funny. The first time Mr. Bushido appears it's hard not to laugh at how hysterical he's become (Tropes--even this one--Are Not Bad after all). But it's still Character Derailment.
His derailment does have a partial explanation. He shows small hints of his otaku ways all throughout Season 1, but he's otherwise normal. In Season 2, whenever he's not fighting he does have moments where he does act normal, even telling Billy the "Mr. Bushido" moniker was not one he was purposely angling for, and when he meets up with Setsuna it's flatly obvious the man has been severely depressed for awhile and he reveals he immersed himself in his Japanese otaku ways as something of an escape from the pain. Unfortunately, the derailment arguement still holds water because the show does a poor job of explaining the whys and when of his plunge into Occidental Otaku.
Nena Trinity got derailed during the 2nd season as well. After beginning to develop standards and generally become less psychotic, a one-year timeskip occurs and Nena is suddenly right back to her previously evil, batshit insane self with no explanation whatsoever given. And then she dies.
The sick thing is that wasn't the only SEED/SEED DESTINY expy derailment that 00 performed (and the fanbase blatantly ignored). Setsuna could be considered one such example; he went from being a Gundam Meister who wanted to change the world for the better (yet ended up doing more damage and finding himself questioning it for a time) into a more literal incarnation of "Jesus Yamato", complete with Messianic superpowers, the ability to make both sides come to an understanding by inducing forced telepathy and above all else being "unquestionably right" always. The latter two were especially played out to the extreme in the movie. Funny how he wasn't renamed "Jesus Setsuna", even though he was much closer to being a "savior" (such that he was even referred to as such by other characters) than Kira ever was.
Kaede Fuyou from the anime SHUFFLE! Is best known for going Yandere when the male lead chose another girl over her. If you described this to someone who had only read the Visual Novel that the series is based on, they would be very confused, since her behavior during the exact same situation in the game was about as far from 'Yandere'as it is possible to get.
A case can be made for more than a few of the characters of Code Geass, but none more so than Ohgi, initially the voice of reason of the rank-and-file Black Knights who understood the burden of leadership that rested upon Zero, he lost the plot once he fell for Villetta and made many questionable decisions, with ultimately disastrous results.
A Certain Magical Index's Kuroko Shirai zig zags this. On regular plot (Both for Index and Railgun), she's shown as a competent member of Judgement and a reasonable authority figure that just happens to have a crush on a girl, which is Played for Laughs but is hardly the only thing she does. On anime-only stories, she suffers from full-blown Character Exaggeration and becomes a full-blown Psycho Lesbian who's willing to resort to drugs to get Mikoto, which she (thankfully) fails at. And yet the second mode is much more popular.
Which is an example that Tropes Are Not Bad which, while the behavior and the Flanderization are disturbing, does make Kuroko a more interesting character if at cost of some of the show's morality, which is hardly that bad in light of the fact that show is quite lacking in it in various areas anyway.
Baka to Test to Shoukanjuu did this to poor Mizuki Himeji. In the first season she was a nice, sweet girl, with one scene hinting at Yandere-ish Hidden Depths but mostly as an one-off gag and nothing serious. Come the OVA and second season, she's basically a clone of the the much-ruder Minami (Who herself suffered Flanderization and became much worse than she was) only with longer hair and bigger breasts. Thankfully, the last episodes seems to bring her back to rails, while also smoothing over Minami a little.
Kanade and Usami are both getting hit by this on Mayo Chiki. Kanade is going from a sexually dominant, but playful and overall nice girl to a JerkassTroll that has no problem on causing property damage and intimidating others with her Battle Butler. Usami went from a nice Tsundere to a generic angry tsundere, overlapping with Character Exaggeration. Kanade eventually gets better, Usami.... is Demoted to Extra so nobody knows.
However, both pale compared to Unlucky Everydude Kinjirou in the Gecko Ending. To summarize, the entire show is about him having gynophobia (fear of women) and the other girls trying to help him with that. Unlike some harem mains, he's not a pervert nor anything like that, and in fact the girls are worse than him. But in the Gecko Ending, he suddenly becomes a very generic pervert that nosebleeds when he sees Usami's panties and can't stop thinking of having sex with Subaru. Which is the exact opposite of how he normally is. He also gets Charles Atlas Superpower out of freaking nowhere.
Tails from Sonic X. In the first two seasons, while there was some heartwarming bonding between him and Chuck, he was characterized as being someone extremely intelligent who happened to be a kid, rather than a kid who happened to be extremely intelligent as he is characterized in the games. He becomes derailed even more in the third season.
In Puella Magi Madoka Magica, Kyosuke Kamijou (Sayaka's Childhood Friend love interest, who seems to think his relationship with her as strictly platonic) and Hitomi Shizuki (one of Sayaka's friends and The Rival for Kyosuke's heart) are probably the most hated characters in the series(after Kyubey) for sending Sayaka on her Start of Darkness. However, their actions in the anime are somewhat understandable and they do give Sayaka some Hope Spots (e.g. Hitomi tells Sayaka to face her feelings and gives her a day to confess to Kamijou). [[spoiler:In the last few episodes, after Sayaka's death, it is revealed that Hitomi fell into depression and thinks it's all her fault while Kamijou also seems to miss Sayaka. Unfortunately, these shades of gray are all dumped in the PSP game. Hitomi becomes a full-on AlphaBitch in Sheep's Clothing, while Kamijou, sees Sayaka start decomposing due to her distance from her Soul Gem and calls her "a disgusting (understandable) monster (not so much)", then twists the knife further by adding "Shizuki is the only one I need!" Ouch.
Goku's wife Chi-Chi is introduced as a strong, capable fighter in her own right, even convincing Goku to agree to marry her as a result of the fierce fight she puts up in the 23rd Tenkaichi Budokai. Come Dragon Ball Z, and she's basically retired from fighting to become an overprotectiveEducation Mama (with severely Skewed Priorities and Insane Troll Logic to boot). While it doesn't excuse her Skewed Priorities, Insane Troll Logic and some of her other undesirable traits, this may be somewhat justified. This is because dealing with your family members being constantly put in danger will drive even the most rational parent insane. It doesn't help that Goku was killed and Gohan was kidnapped and subjected to brutal Training From Hell by Piccolo. The Majin Buu Saga fixes thissomewhat.
Unfortunately, Dragon Ball Super not only reverted Chi-Chi right back to her old ways, but made her much more worse. In DBZ, she seemed to mellow out after Cell killed Goku, even training their second son Goten in martial arts personally. In Super, not only does she start displaying the same Education Mama attitude towards Goten, but she's also gotten even more overprotective of her family members. She even goes as far as to kidnap Gohan and Videl's newborn daughter Pan just because Gohan and Mr. Satan discussed the possibility of her becoming a Z-Fighter when she grows up.
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