Various characters from The Fairly OddParents! underwent this trope during its run from the original Oh Yeah! Cartoons shorts to its tenth and final season.
- During the first few seasons of the show, Timmy was mischievous, yet essentially a good kid who occasionally gave into his selfishness. Subsequent seasons would later play his meaner side more often, but he retained his good heart. However, by about season 5, he became full-on mean along with all the other denizens of this hellish parody of sitcom America, including his godparents who were summoned to help him in the first place. He returned to most of his original affable characterization by the sixth season, but it still shows up every now and again. He relapses back into it in the ninth season. In the case of Season 10, it's so his role as Chloe's Foil can be made more apparent.
- Cosmo, in the original Oh Yeah! Cartoons pilot, was actually smart, suave and sophisticated, barely resembling the character he became. By season 1 characterization marched on; Cosmo became a bit of a Cloudcuckoolander, sometimes dim but still relatively normal and well-meaning, and perhaps strangest of all, he and Wanda seemed to be on an equal level of airheaded and careless intelligence ("We're two halves of a whole idiot!"). Starting around season 3, he started to become increasingly dumber with each season, going from a Genius Ditz, to just The Ditz, to even Lethally Stupid. His relationship with Wanda in seasons 4 and 5 became the foil for a lot of marriage jokes; while the pilot episodes have them be a lovey-dovey couple, in some later episodes, he claimed to be trapped in the marriage to Wanda and that she had tricked him into it. This characteristic was toned down in Season 6, when the birth of Poof turned them into loving parents.
- Wanda was originally portrayed as being dimmer than Cosmo, believe it or not, then at around season 1 they shared an equal amount of intelligence. From season 3 onwards, Wanda became simply the wiser one who chastised Timmy and Cosmo whenever they did something wrong. In subsequent seasons all her jokes tended to be that she's a total nag; this was lampshaded in the sub-plot of "Nega Timmy", when she had to prove to Cosmo she can resist her urge to call the two out when things go wrong. This reached its culmination when the episode "Timmy the Barbarian" had her become the whip cracking Nagules! While still remaining the voice of reason, the jokes on her being a nag were toned down from season 6 onward.
- Timmy's parents were initially just busy and clueless (simply a part of Timmy's Hilariously Abusive Childhood), until season 5 onward makes them extremely negligent parents who lampshade said negligence and have even hinted at times that they don't truly love Timmy, seeing him as a burden. This was actually the plot catalyst in "Nega-Timmy", which featured a parody of the late Steve Irwin as 'The Bad Parent Hunter'.
- Vicky was originally just apathetic to Timmy and more complacent with sitting around watching TV; in fact, in the early episode "The Big Problem", Vicky freaks out when Timmy goes missing under her care — sure, only out of concern for herself, but at least she intended to do her job correctly. In subsequent seasons she became a Babysitter from Hell, blackmailing Timmy into doing chores by revealing false evidence that he cheated on a math test or wrecking the house while his parents were out. Starting around season 4, however, she wants to Take Over the World, is ferocious enough to intimidate and whip all the other villains into shape, goes out of her way to harm Timmy and does little to cover up her actions because no one will stop her anyway.
- Jorgen Von Strangle, like Cosmo, went through two Flanderizations. He began as a stern Terminator parody, and had little real personality apart from brutally hurting other characters and destroying things whenever he is set to do a job. Somewhere in season 1, he has more of a personality. He becomes the boss of all fairies (which was undetermined before, and he was only assumed to have every job a higher official could have), so now he has a reason to brutalize the other fairies, with monstrous power and a very well defined and muscular body compared to all the others. His first Flanderization was in subsequent seasons where he started to show much more sadism and enjoyment in beating, humiliating, and gloating in front of all the fairies. But in season 7, he went through his second Flanderization; he becomes a Plucky Comic Relief character who never uses his power in dire times, only his body. It's as though he isn't even a fairy anymore, or the boss of fairies at that. If this weren't noticeable enough, he becomes a perfect ally and even a dear friend to Timmy. They were always his closest friends, but now he doesn't even punch Cosmo anymore.
- Trixie Tang started as a Lovable Alpha Bitch, antagonizing Timmy but at times showing a nice side to her, as well as a tomboyish side. Starting in season 3, her tomboyish side just disappeared, and she lost her softer side, becoming just a stereotypical Alpha Bitch.
- Tootie at first only had an "innocent schoolgirl" crush on Timmy. In later seasons her crush evolved into an obsession. But by season 4 or so, it evolved into outright stalking; now she wants to get a law made that would make him her property, and when she's cheering him at one point mentions she stalks him every night. While not truly evil, she's become almost as bad as her sister Vicky, and it does not have to be the violent way.
- While Mr. Crocker was always a bit crazy, in the earlier episodes his usual demeanor was more stoic and grouchy, only veering into maniacal insane behaviour when there were fairies at work. A lot of his insanity was perceived by the other characters due to him being The Cassandra, and his fairy hunting tools were more grounded in reality (for this show, anyway). He was also either snarky or silent when he gave kids failing grades. Starting in season 3, he has a huge lab underneath the school (to the point where it's lampshaded) and he's every bit as crazy as everyone perceives him to be. Sometimes these days it seems like he can't even form a sentence without "FAIRY GODPARENTS!" in it somewhere. He also became MUCH more fond of giving students failing grades, and it's one of the only things that makes him happy; hell, in his first appearance, he didn't even give out Fs, but Ds. In Season 10, he undergoes another Flanderization; while he's still as manic as ever, his obsession with Fairies, his defining trait since his debut, is completely gone (unless the episode's plot requires it) to the point that he doesn't even spaz out when he notices anything magical nor does he even say his Catchphrase anymore.
- AJ was originally just the smartest kid in Timmy's class. Now he rivals Jimmy Neutron in terms of brains — a connection that was not lost in the third Jimmy/Timmy special.
- Chester was originally just a lower-income kid who lived in a trailer and whose father was a disgraced baseball player. Now he eats garbage and such things with so much vigor you wonder what he would do if he were rich. Also, near the beginning he appeared more down to earth in comparison to AJ, but now he's just a Dumb Blonde.
- Every character since Season 9 onward has been getting progressively dumber. In the Season 9 episode "Let Sleeper Dogs Lie", there are jokes that make even Wanda seem dumb. The characters become even dumber still in Season 10, including Wanda, and Chloe is usually the Only Sane Man and the one who ends up saving the day.
