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* Sakura Kinomoto of ''Manga/CardcaptorSakura'' plays with this. In early episodes she was something of a NaiveEverygirl, childlike but fairly normal in terms of her age's intelligence and demeanor. In later episodes she is something of a GeniusDitz, highly developed in her handling of her cards and even the odd bit wisdomic, but an exagerratively naive KindheartedSimpleton in most other areas.
* ''Anime/CodeGeass'': Ohgi, after falling for Villetta. It's so bad that thanks partly to him, [[spoiler:Lelouch finishes crossing the DespairEventHorizon]], which causes [[spoiler:Lelouch]] to implement the [[spoiler:[[ThanatosGambit Zero Requiem]]]].
* ''Manga/DeathNote'':
** Light Yagami. He goes from erasing his own memory, trusting in the brilliance of his planning to ensure L's death while cementing his own complete innocence... to hiding the Death Note in storage and ''really hoping no one finds it while he's not looking''.\\
The whole Mello and Near arc has Light {{nerf}}ed down hard, making great mistakes apparently to balance the odds of the two kids-nemesis. He loses the note, he loses Sayu, he loses his father... and one of his "best" responses is to confirm to Near he's Kira by ''sending a mass of stupid people at the exact location (and '''only there''') of Near's anti-Kira organisation with the '''great''' expectations that they'll lynch someone of L's level''. An extended period of complacency between arcs with no intellectual rival, and belief in his own [[AGodAmI hype]] likely have something to do with it, as does the fact that Near and Mello have far more information to work with than L did.
* ''Anime/DragonBallZ'': Before the Cell Games, Cell was TheChessmaster and a NoNonsenseNemesis who constantly ran circles around the Z-Fighters and outsmarted them on more than one occasion. But come the Cell Games, he's an arrogant, egotistical jackass of a SmugSuper who makes the ''exact same mistakes'' he had previously mocked Vegeta and Trunks for making. When [[IdiotHero Goku]] of all people is able to hit you with a BatmanGambit, you ''know'' you've latched onto the IdiotBall.
** Speaking of Goku, he qualifies in ''Anime/DragonBallSuper'' (relative to ''Anime/DragonBallZ''), as, while he's always been an IdiotHero, ''Super'' puts much more emphasis on the "Idiot" part. He's portrayed as more of a ManChild who doesn't care about much of anything aside from his desire to [[BloodKnight fight strong opponents]] and is willing to risk everyone else's well-being to do so which goes as far as the ''entire'' universe just to fight in the tournament. However, WordOfGod is that ''Dragon Ball Z'' was actually an example of AdaptationalHeroism and this is closer to how Goku was originally meant to be portrayed.
** [[spoiler:[[FusionDance Merged Zamasu]]]] ends up being this compared to his [[spoiler:fusees]] [[EvilTwin Goku Black]] and [[CompleteImmortality Future Zamasu]]. While [[spoiler:Merged Zamasu's]] by far even more immensely powerful than either of them are, he lacks many of their [[CombatPragmatist pragmatist]] traits that made the two so dangerous to the heroes. Granted, they could be arrogant as well, but even Future Zamasu who abused his immortality for all its worth knew the value of fighting in tandem with Black and would even [[StoneWall use himself as a shield to keep unnecessary harm from hitting his partner]]. [[spoiler:Merged Zamasu]], on the other hand, rather than finishing the Z-Fighters off at the first opportunity he chose to toy around with them and tends to use his techniques just to show off. He also kept underestimating his enemies and seemingly cannot accept that the "lowly mortals" somehow manages to even scratch him. This arrogance eventually lets them destroy his physical body. [[spoiler:His EldritchAbomination is this even more as his madness has practically consumed his mind to the point he's barely sentient anymore]].
* Alphonse in [[Anime/FullmetalAlchemist the 2003 anime version]] of ''Manga/FullmetalAlchemist''. He starts off as an intelligent, insightful young boy who was the [[StraightMan sensible one]] compared to Ed. Later on he makes many very poor, and arguably stupid choices like taking [[KnifeNut Barry the Chopper's]] mind games to heart, and distrusting people who he's known all his life. This happens in both the manga and anime, but in the manga, Al learns from this and doesn't do anything like it again. In the first anime it mainly takes effect in the later half,where he runs off every time he gets upset, even though the Homunculi [[spoiler: and Dante]] are looking for and trying to kill them, going along with Shou Tucker's plans and not telling Ed (even though Tucker made his daughter [[spoiler:into a chimaera, resulting in her death]], and letting Sloth convince him that she's his mother, even though she is a Homunculi when Ed is trying to remind him that they're fighting for their lives, [[spoiler:immediately running towards Envy disguised as Winry, right after Ed says it isn't her, and barely putting up a fight.]] He arguably became more impulsive than his brother, whereas earlier in the show it was the other way around. This could be interpreted as the writers having Al act more like how a real little boy would if they were put in such a situation, [[spoiler:but he becomes more competent again in the finale]]. Other than that he basically becomes [[TheLoad quite the load]].
* ''LightNovel/{{Kampfer}}'': Natsuru but only in the anime. In the manga and light novels [[CrouchingMoronHiddenBadass however...]]
* Whenever ''Manga/{{Naruto}}'' seems to start growing as a character in-canon, he immediately takes a level in dumbass in the anime {{Filler}}. Most notoriously when he doesn't realize anything is wrong during the attack on turtle island by Kabuto and zombie!Deidara. Despite the living island suddenly acting as if it were hit with a high-level earthquake.
** For a long while, Sasuke from [[spoiler:the death of Itachi]] and throughout the succeeding arcs. Even after both Tobi and Itachi had both implored that the destruction of Uchiha clan wasn't the fault of the Leaf, he decides to screw everything and try to destroy the Hidden Leaf village and every single person living in it, despite Itachi's intentions and words told by the main antagonist of all people himself urging the very contrary. All the while, his chakra grows noticeably darker and colder through his hatred, implications seeming as if he's forcing himself to hate in order get more powerful. At one point, he impales [[spoiler:Karin]] when held hostage with the Chidori, despite Danzo holding her in a position where his body and head is in clear sight of his lightning-fast attack and discards her for being "useless".
* Arguably Luffy of ''Franchise/OnePiece''. He was never a genius by any account, but he seems to have gone from "not bothering to understand what people say" to "not able to understand what people say" in some cases. He also showed at least some insight at the start of the series, like realizing how he could make Coby join the Marines and calling Vivi out on how it's naive to hope that nobody will die in a war. He has not really had any of those moments for a long time. However, it may be justified in that Luffy is not good at ''anything'' but fighting and socializing. At the start of the series, he had a very small crew and thus had to handle several tasks himself. Nowadays, his bigger crew of eight members handle all the practical things for him, and in return he beats all the bad guys they don't have a chance against. So when he does not ''have to'' use his brain, he probably has made it a habit to switch it off most of the time. It's noteworthy that when the crew was split up, and he was on his own, he did seem a bit more intelligent and thoughtful, but when he gathered some new allies who could do the thinking work for him, he returned to his usual ditzy self.
* ''Anime/{{Pokemon}}'': The Team Rocket trio. In the early appearances they are very competent, albeit very hammy, and have succeeded several times in stopping the heroes and capturing Pikachu only to be save by a BigDamnHeroes moment or an AssPull of some sorts. Fast forward and their only real purpose now is to show up, provide comic relief, and blast off again, barring the episodes that focus on them being sympathetic to the point where they might as well not be villains if not for [[StatusQuoIsGod status quo getting in the way]].
** This is finally subverted in Best Wishes as they now TookALevelInBadass.
*** [[DoubleSubversion However, they eventually reverted to being comic relief villians.]]
** Ash himself has zigzagged this trope and DumbassNoMore throughout most of the seasons; putting the fully experienced, competent and intelligent trainer he ends most seasons as into the next season - where he's expected to go through the early rookie stages again - would kill all drama as he would [[CurbStompBattle utterly destroy the Gym Leaders and rivals]]. Ash deposits not only his Pokemon but his brain cells at Professor Oak's house and goes off to the next region to make mistakes a five-year-old would avoid.
* ''Manga/RanmaOneHalf'': Genma. At the beginning of the series, he's lazy, doesn't think things through, and isn't the brightest bulb on the circuit, but is for the most part a competent martial artist who, despite some (admittedly quite serious) blunders, has trained his son to be an incredibly skilled martial artist. As the manga goes on, he becomes increasingly stupid and incompetent, with only one bright spot during the Ryu arc.
* Sailor Venus from ''Anime/SailorMoon''. See {{Flanderization}}.
** Actually worse in the English dub as they removed her character quirk of mixing her proverbs up in the first season and give her a voice actress with a leader style voice then Sailor Moon S came where Venus's ditzy personality had completely taken over and the English dub just so happened to give her a new voice actress that emphasized this personality.
* Gourry from ''LightNovel/{{Slayers}}'' degraded from IdiotHero to TheDitz and then became even dumber.
** The degradation arguably occurred before that, considering how Gourry's dumbass swordsman persona was apparently [[ObfuscatingStupidity all an act]] in the original novels.
* ''Anime/TenchiMuyo'': Mihoshi, ''especially'' in ''Anime/TenchiUniverse''. Somewhat justified in the case of ''Tenchi Universe'' (and ''Tenchi in Tokyo'') since those aren't actually the same Mihoshi, due to those being [[AlternateContinuity alternate continuities]]. Totally unjustified in the 3rd installment of the OVA series, though.
* In the original series ''Anime/{{Yatterman}}'', Gan is somewhat naive, but not stupid. The remake makes him a bit more careless but still rather smart. The live-action movie, however, turns him into a total IdiotHero.