Trent. In Total Drama Island, he was "the Cool Guy" and the closest thing to a normal teenager: friendly, easygoing, grounded, humble, and talented. He developed a thing for Gwen and they end up going out on the last episode. By the fifth episode of TDA, his relationship with Gwen has turned him into a clingy, neurotic, and desperate loser who has to please her since he's so worried about her feelings dwindling since she has more in common with Duncan. It gets so bad that he starts throwing challenges for her and Gwen has to convince his team to boot him off. And that's not even getting into his sudden maniacal Obsessively Organized obsession with the number "9".
Geoff and Bridgette both completely lost their personalities in the beginning of TDA. Their relationship transformed from ordinary teenage lovers with a supportive emotional connection to each other and many major friendships with other contestants to being excessively physical and hormonal Makeout Kids who spend the other half of their time bickering with each other and were basically Satellite Characters to each other (little wonder the popularity of the "Gidgette" ship massively declined after TDI). However, they for the most part, return to their old personalities as Aftermath hosts in TDA and TDWT.
In TDA, Courtney went from an uptight and bossy but well-mannered and well-meaning Good Counterpart to Heather who had a sense of honor in playing the game to a violent, spoiled, manipulative, and cutthroat Jerkass who sues if she doesn't get what she wants (something she only did once in TDI, in a justifiable case) and cares only about what she wants, becoming pretty much as bad - or worse - than Heather was. And then things got worse after Duncan cheated on her with Gwen in TDWT (done solely to kill DuncanXCourtney and make then-Fan-Preferred Couple DuncanXGwen canon). Being furious and upset over the incident at hand is one thing (and completely understandable), but actually losing challenges on purpose for no reason other than to spite Gwen is worse.
Things just get worse for Courtney in TDAS. At first it appeared as if the writers were finally trying to get Courtney's character back on track. After the whole Duncan incident, Courtney finally became Gwen's friend again and the two were just as close as ever, and she even pursued a new romance with Scott. It looked as though Courtney finally had a chance to be likable again. All that development was suddenly derailed in one entire episode. Out of nowhere, Courtney reveals that she has already planned out what she's going to do now that she's in the final five and what order she plans on getting rid of the other contestants, two of whom are her close friend and new love interest. She chooses to backstab them because a million dollars is worth more than her relationships with either of them apparently.
Happens to Gwen in TDWT, where she goes from a moody but loyal and kind-natured Misunderstood Loner with a Heart of Gold to being an selfish, unrepentant, and manipulative jerkass. She kisses Duncan while he was still with Courtney. She feels some remorse for doing it at first but when the kiss is found out, she doesn't try to vote herself out like the last season or apologize. And later she manipulates Cody's feelings for her to vote Courtney off. She's even lampshaded as being "New Heather".
It only got worse for Gwen in TDAS, with many seeing her as having become "too girly" and too concerned with what other people thought of her (such as giddily gushing with Courtney about each other's hair and clothes), as well as her handling of her second fallout with Courtney, which made her out into a hypocrite for treating Courtney the exact same way she had been treating her in TDWT. It should be no surprise at this point that she has gone from one of the show's most universally loved characters to one of its most divisive.
Leshawna, most definitely. TDI had her as a girl who would take crap from nobody and could get quite feisty, but was still very kind, loyal, caring, and friendly, to the point where her "elimination" came in part because everybody who had been eliminated at that point liked her so much. Then comes TDA, where, for absolutely no good reason she manipulates her teammates into giving her a reward by pretending to cry and being sentimental, only to trash-talk them behind their backs. It's even worse in TDWT, in which she's now a massively arrogant jerkass and a gargantuan hypocrite with little to no redeeming qualities, and goes right back to hating Heather even after supposedly making up with her in TDA; even beating Heather up when Heather is trying to warn her about Alejandro's villainy and, after learning that she was right, is still proud of doing it. Basically, much like Courtney, she went from being Heather's moral and ethical superior to being just as bad as her in every way (in fact worse, due to Heather's Character Development around the same time to a minor Jerk with a Heart of Gold).
DJ too. He was introduced as a sensitive, kindhearted, and easygoing Lovable Jock who was capable of being brave, tough, and even mischievous or playful when the time called for it (like overcoming his fear of water to help his team win a challenge, standing up to Chef for forcing him into an illegal alliance, and joking around with Geoff and Duncan). But in TDWT, he's become a gigantic crybaby who Screams Like a Little Girl at virtually everything, and constantly cries and blubbers for his mother (something he never did once in TDI).
Ezekiel's case in TDWT is one of the most infamous in the show's history. Originally a meek and clueless homeschooler who got eliminated for making several unintentionally sexist remarks. When he came back in World Tour, he was turned into a Small Name, Big EgoDeterminator and an obnoxious Jerkass. After losing the first challenge for his team and being voted off, he decided to stow away in the ship's cargo hold, which eventually resulted in him mutating into a feral, barely-sapient GollumExpy with almost no trace of the naive and innocent farmboy he once was left in him.
Duncan in TDAS, has his Hidden Heart of Gold and strong desire to not be seen as soft exaggerated to massive degrees, and his attempts to prove he's still a bad boy fail. It comes to a head when he blows up Chris' house, gets the cops called on him by the host, and gets arrested. As a result, Duncan unsurprisingly became one of the most controversial characters in the whole series.
Lindsay was set to to grow leadership skills and develop some brainpower in TDA, but that was suddenly thrown out of the window with her elimination (in which she voted herself off by accident) and never revisited afterwards. She ends up getting even dumber in TDAS, being unable to understand simple concepts like pushing and pulling in TDAS.
Like all of his team that season, Harold was hit by this in TDWT. In the previous 2 seasons, he was a well-meaning and intelligent Crouching Moron, Hidden Badass whose "mad skills" rescued his team on more than one occasion, thus proving himself a good teammate whose boasts were almost never empty. Suddenly, in TDWT, he's become an unbelievably arrogant Small Name, Big Ego and Know-Nothing Know-It-All who does nothing worthwhile to help his team in challenges (unless constantly prattling on about himself and his "knowledge" is considered something) and whose only contribution is being hit with the Idiot Ball and getting convinced by Alejandro to perform a Stupid Sacrifice.
Sierra, primarily the focus of her fangirlism, as she was initially a fangirl of Total Drama in general who knew most of the tropes and patterns of the series and her obsession with Cody was a minor draw of her fangirl attitude. She also looked up to Chris enough at the beginning of World Tour to name her team "Team Chris Is Really Really Really Hot." Later episodes would drop these aspects and just stick to her obsession with Cody.
From the cast of Revenge of the Island, we have Zoey and Cameron, thanks to their friend (well, boyfriend in Zoey's case) Mike and his Superpowered Evil Side Mal. Zoey went from a grounded but somewhat gullible girl who was able to stand up to herself when needed to being Too Dumb to Live and completely unable to recognize that Mike was actually Mal despite being warned by three different people that Mike was Not Himself (to the point where she actually thought Alejandro was warning her about Courtney in one episode) and being aware of his DID from the previous season. Cameron, despite being established in his debut as one of the show's most intelligent characters bordering on Impossible Genius, stupidly trusted Mike when he was Mal, somehow failing to recognize some of the very obvious signs something was wrong with his friend even though he was one of the first to discover Mike's mental disorder in the previous season.
In TDAS, the snarky, devious Scott has his characterization altered completely into becoming a Half-Witted Hillbilly who gets confused by simple metaphors, with no traces of his strategic self from ROTI remaining. Unlike most cases of derailment though, many fans enjoyed the change in Scott's character due to the humor it brought.
Dave from the cast of Pahkitew Island is a very extreme case as he was derailed in the very season he debuted in. At the beginning of the season, he was established as a pessimistic and neurotic Deadpan SnarkerOnly Sane Man who also happened to have a crush on the friendly and grounded Passionate Sports Girl Sky. But around the 6th or 7th episode he's completely lost all his other traits to focus solely on his crush on Sky, as he started becoming too dependent on her which led to him suffering from an emotional breakdown when she and Max switched teams and ultimately voting himself off in the 9th episode when she rejected him (albeit rudely due to being in a bad mood at the time), leaving him a bawling wreck and beginning a Sanity Slippage. Things got even worse in the finale when he completely loses his mind after learning that the reason why Sky rejected him was because she already had a boyfriend back home (who she tried to break up with prior to joining the show), conducts a huge Face–Heel Turn, and abruptly becomes the Final Boss of the season in order to gain the million for himself.
