Few anime characters display the same personality traits at all times, but not all of them change drastically. In some cases though, the change can be pretty stark. One type of character whose personality does a complete 180 are chilled out, nice anime characters who get serious when they have to deal with conflict.
One example of this trope is Hatake Kakashi of Naruto, who is normally a laid back dude who goes totally serious when fighting against enemy ninja. There's also Rimuru Tempest of That Time I Got Reincarnated As A Slime, who faces everyone and everything with a smile until he's actually staring down somebody who means him harm. Nezuko Kamado of Demon Slayer might be the cutest thing ever, but if you touch Tanjiro she's a force to be reckoned with.
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Gon Freecss is a friendly, adventurous boy who is level-headed, confident, and happy most of the time. However, he has a hair-trigger temper and can become enraged when the people he cares about are in danger. For example, when Neferpitou's actions lead to Kite's demise, Gon is utterly unforgiving, transforming into an older version of himself with enough power to beat Neferpitou into an unrecognizable pulp. Gon can be a sweetheart, but when things get serious, he can be remarkably cold and even cruel.
On the outside, Meliodas is a calm, funloving person who loves telling jokes and generally having a good time. But if anybody lays a finger on any of his friends - especially Elizabeth - he instantly goes into Berserker-mode. Unlike some characters on this list, Meliodas' rage was actually a dominant trait for much of his life - in the past he was a terrifying demon known for his extreme cruelty. Nowadays, he's trying to be a lot more low-key, but that's not always easy to do when you're literally the Sin of Wrath.
When Kakashi was a kid, he was a super serious person who was hardened by his traumatic experiences as a child soldier. He had little patience for nonsense, especially when it came from his more boisterous teammate, Obito. He was fully dedicated to his obligations as a ninja, and had no time for anything else.
As an adult, Kakashi has calmed down a lot. In fact, most of the time he's an amicable, friendly dude who can sometimes be found reading mildly salacious books for fun. But his cheerful nature instantly flies out the window when he's facing down an enemy ninja. His formerly serious personality comes out, and he uses anything from his Chidori attack to his mangekyo sharingan to do his duty as a ninja.
Saitama is just about the most OP character in the anime universe - as long as he can land a hit, he can knock out any opponent with a single punch. While it does take some work for him to maintain that strength, he doesn't let all that power go to his head. Actually, he's a totally laid back dude who spends most of his time chilling out at home playing video games or collecting coupons to lower his grocery bill.
But when actual danger comes calling, he slides seamlessly into superhero mode, and knocks his enemies into the stratosphere.