Character derailment

Character derailment is the opposite of mh:greatcharacters:character development. Instead of learning from mistakes, characters just get worse.

It is not to be confused with flanderization, as flanderization involves over-exaggerating and amplifying a character's traits. Character derailment, on the other hand, refers to a character's traits becoming vastly different compared to what was previously shown.

Notable Characters Who Suffered Character Derailment
During his introduction in Season 4, he was treated like an old, wise engine. But in Seasons 8-12, do you even call him a realy useful engine in the Skarloey railway? Thankfully in the Movie "Blue Mountain Mistery", he is back portrayed as and old grandpa.
 * Fonzie (Happy Days; Season 4-11; He is the trope namer for jumping the shark, which he literally did in the episode Hollywood Part 3. This is character derailment because in Season 2 he got hurt)
 * Air Conditioner (Wolfoo; he was nice to Wolfoo and Lucy for a short time but then he treats them like crap)
 * Skarloey (Thomas and Friends):
 * Kai (mh:besttvshows:LEGO Ninjago: Masters of Spinjitsu; Seasons 8-12)
 * Henry Hart/Kid Danger (mh:terribletvshows:Henry Danger)
 * Ray Manchester/Captain Man (Henry Danger)
 * Charlotte (Henry Danger)
 * Jasper (Henry Danger)
 * Piper Hart (Henry Danger)
 * Danger Mouse (2015) (mh:besttvshows:Danger Mouse (2015))
 * Sasuke Uchiha (Naruto Shippuden)
 * Madara Uchiha (Naruto)
 * Yuuki Terumi (BlazBlue series)
 * Nana (Elfen Lied)
 * Son Goku (Manga only)
 * Dongtae (DICE: The Dice That Changes Everything; season 4)
 * Aya Asagiri (Magical Girl Site; second half-became a muted girl after Yatsumura lost her lifespan and vanished)
 * The Thunderman Family (The Thundermans)
 * Thomas the Tank Engine (Seasons 12-16)
 * Dawn Harper (Nicky, Ricky, Dicky, & Dawn)
 * Daffy Duck (Looney Tunes; DePatie Freleng/Seven-Arts Era)
 * Spongebob Squarepants (SpongeBob SquarePants: Seasons 6-8)
 * Patrick Star (Spongebob Squarepants; Seasons 6-8)
 * Mr. Eugene Krabs (Spongebob Squarepants)
 * Cosmo Cosma (The Fairly OddParents; Season 9-10)
 * Carl Wheezer (The Adventures of Jimmy Neutron: Boy Genius)
 * Sheen Estevez (Planet Sheen)
 * Seymour Skinner (The Simpsons; Seasons 9-30)
 * Discord (My Little Pony: Friendship is Magic; Season 9-He had a chance of becoming a good guy, until it was revealed that he was Grogar all along. He then put the whole world in danger as he made the terrible trio so powerful that even he couldn't deal with them, which just goes against his whole development)
 * Stella (Winx Club; Seasons 5-9; Her childishness got much worse, with her throwing actual tantrums over not getting her way.
 * Diaspro (Winx Club; Seasons 3-9; Was derailed into a Miss Jerkass Yandere who eagerly brainwashed Sky in order to marry him and tried to kill Bloom. She was also made a Karma Houdini because she wasn't punished for what she had done at Domino.
 * Tecna (Winx Club; Seasons 5-9; She went back to acting like her season 1 self, thus losing a lot of the development she got in seasons 1-4. She still had several good moments in the later seasons though)
 * Chloe Bourgeois (Miraculous Ladybug; Season 3-present)
 * Eddy (Ed, Edd, n Eddy; Season 5-6)
 * Ed (Ed, Edd, n Eddy; Seasons 5-6)
 * Sarah (Ed, Edd, n Eddy; Seasons 5-6)
 * Jimmy (Ed, Edd, n Eddy; Seasons 5-6)
 * Kevin (Ed, Edd, n Eddy; Seasons 5-6)
 * Rolf (Ed, Edd, n Eddy; Seasons 5-6)
 * Johnny 2x4 (Ed, Edd, n Eddy; Seasons 5-6)
 * Lee Kanker (Ed, Edd, n Eddy; Seasons 5-6)
 * Marie Kanker (Ed, Edd, n Eddy; Seasons 5-6)
 * Numbah Four (Codename: Kids Next Door)
 * Hot Shot (Transformers: Cybertron)
 * Danny Fenton/Phantom (Danny Phantom; Season 3)
 * Inferno/Roadblock (Transformers Energon Season 3?)
 * Bumblebee (Transformers: Robots in Disguise 2015)
 * Ellie (The Last of Us Part II)
 * Lynn Loud Jr (The Loud House)
 * Light Yagami (Death Note)
 * Suzaku Kururugi (Code Geass)
 * Nino Arizugawa (Anonymous Noise; Had been forming a relationship with Yuzu (even being kissed by him very often), only for her to turn him down and go back together with Momo.)
 * Sindel (Mortal Kombat 11)
 * Waldek Kiepski (Świat Według Kiepskich)
 * Jin Kazama (Tekken 6)
 * All of the main characters (Game Shakers)
 * Harry Potter (Harry Potter; when he watches in horror from Cedric's death)
 * Johnny Test (Johnny Test 2005; usually seen as selfish, disrespectful, immature, gross, and narcissistic. While he was slightly tolerable in the first season, he got flanderized by the second season and started to mess up his sisters' inventions for his own self-serving purposes, which resulted in him causing trouble for himself and the whole city.)
 * Lincoln Loud (The Loud House; Season 4-present; he did very stupid and selfish things he has done.)
 * Gelatin (Battle for BFDI; While he has always been impulsive to some extent, this is cranked up to 10 in this season where he makes mistakes and act reckless even more often)
 * Memetchi (Tamagotchi! Season 1B)
 * NostalgiaSquare (2018)
 * Bob (2018)
 * Bowser (SMG4; 2018-present; He started to go from a villain like his original counterpart, to a friendly hero who is friends with Mario, SMG4, and the gang and rarely kidnaps Peach. In 2019, he began to suffer from being demoted to a recurring/minor/background character like all the other old characters whether they are Mario characters or not who are not Mario, SMG4, Luigi, Fishy Boopkins, Bob, Chris and Swagmaster, SMG3, or Meggy and Shroomy to lesser extents presumbly due to the popularity and shoehorning of the OC characters like Tari, Saiko, Axol, and Melony.)
 * Bella Swan (Twilight)
 * Sam Puckett (Sam & Cat)
 * Any character from Sunny Bunnies (Season 4-present)