Took A Level In Dumbass / Anime & Manga

• Sakura Kinomoto of Cardcaptor Sakura plays with this. In early episodes she was something of a Naïve Everygirl, childlike but fairly normal in terms of her age's intelligence and demeanor. In later episodes she is something of a Genius Ditz, highly developed in her handling of her cards and even the odd bit wisdomic, but an exagerratively naive Kindhearted Simpleton in most other areas.
• Code Geass: Ohgi, after falling for Villetta. It's so bad that thanks partly to him, Lelouch finishes crossing the Despair Event Horizon, which causes Lelouch to implement the Zero Requiem. Assuming this still happens in the Compilation Movie, it may overlap with Dumbass Has a Point, Character Rerailment, or both when he actually attempts to give Lelouch a chance this time, and is simply ignored.
• Death Note's 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 nerfed 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 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.
• Dragon Ball:
  • Before the Cell Games, Cell was The Chessmaster and a No-Nonsense Nemesis 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 Smug Super who makes the exact same mistakes he had previously mocked Vegeta and Trunks for making. When Goku of all people is able to hit you with a Batman Gambit, you know you've latched onto the Idiot Ball.
  • Speaking of Goku, he qualifies in Dragon Ball Super (relative to Dragon Ball Z), as, while he's always been an Idiot Hero, 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 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. It really doesn't end there however: in the manga's Galactic Patrol Prisoner saga, despite Jaco's pleas to just Finish Him!, Goku gives Moro - a guy who has driven the Namekian race to near extinction and has been stated to have consumed numerous planets before this - a Senzu Bean, as well as the stupidest reason for doing so: he wants Moro to go back to prison (which has Goku - the protagonist we're supposed to be rooting for - verging on the point of Too Dumb to Live). Additionally in The Movie Dragon Ball Super: Super Hero, Goku ridicules Vegeta for meditating and Whis lectures him that there's more to training than bulking up your body, but we’ve seen him practice meditation himself countless times before this.
  • In the Namek Saga, Vegeta comes across as one of the smartest characters in the conflict: he's regularly shown manipulating his opponents, coming up with fairly good plans, and being pragmatic in how he operates. He would retreat from fights or avoid conflicts if it didn't seem like he could win them, had no problem abusing external or "unfair" sources of strengthening, and more than once teamed up with people in fights when he needed to—though he could be overcome by arrogance, more often than not he clearly understood the weight of the situations he was in and acted accordingly. During the Cell and Buu Sagas, however, this flies out the window, and Vegeta becomes seemingly incapable of considering the consequences of his actions or coming up with any plan more sophisticated than overpowering the opponent. Over the course of the two arcs, he treats the idea of quickly and pragmatically dealing with a lethal threat as cowardice, goes out of his way to strengthen his opponents if he thinks they can give him a good fight, and rejects the idea of fusing with Goku even in the face of insanely strong enemies.
  • Merged Zamasu ends up being this compared to his fusees Goku Black and Future Zamasu. While Merged Zamasu's by far even more immensely powerful than either of them are, he lacks many of their 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 use himself as a shield to keep unnecessary harm from hitting his partner. 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. His Eldritch Abomination is this even more as his madness has practically consumed his mind to the point he's barely sentient anymore.
• Alphonse in Fullmetal Alchemist (2003). He starts off as an intelligent, insightful young boy who was the sensible one compared to Ed. Later on he makes many very poor, and arguably stupid choices like taking 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 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 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, 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, but he becomes more competent again in the finale. Other than that he basically becomes quite the load.
• Kämpfer: Natsuru but only in the anime. In the manga and light novels however...
• Naruto:
  • Whenever the title character 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 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 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".
• Luffy of One Piece 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. These moments become rarer as the series goes on. However, 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.
• Pokémon: The Series:
  • 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 Big Damn Heroes moment or an Ass Pull 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 status quo getting in the way.
  • Ash himself has zigzagged this trope and Dumbass No More throughout most of the seasons; since the anime is used as a Gateway Series for the games themselves, they often have to explain the mechanics again at the start of each arc for those just jumping on, as well as give the new-gen Pokemon spotlight. This results in each region Ash depositing his previous team to start over fresh, though he generally keeps to the level of a (maybe above) average trainer starting off, especially if he has a newbie traveling companion he would have to teach the basic ropes too. Pokémon the Series: Black & White is the worst example by far, however, and the one most people think of as being the standard for all regional anime starts, as in addition to replacing his team, Ash ends up losing the skills he was generally allowed to keep in most regions to go back to the same level of competence he had in Kanto to allow the writers a way to explain the game mechanics again.
  • One of the worst offenses was during his battle with Electric Gym Leader Elesa, also in the aforementioned black and white series. He hoped to power through her entire team with just one Pokemon, he decided to bring just that. When it fails, he stalled the battle to retrieve another. Without a plan, it ends badly and causes him to panic and risk forfeiting his match. It took Pikachu to literally zap him to his senses and help win.
• Ranma ½: 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 Sailor Moon. 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 Slayers degraded from Idiot Hero to The Ditz and then became even dumber. The degradation occurred before that, considering how Gourry's dumbass swordsman persona was apparently all an act in the original novels.
• Tenchi Muyo!: Mihoshi, especially in Tenchi Universe. Somewhat justified in the case of Tenchi Universe (and Tenchi in Tokyo) since those aren't actually the same Mihoshi, due to those being alternate continuities. Totally unjustified in the 3rd installment of the OVA series, though.
• In the original series 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 Idiot Hero.

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