Katekyou Hitman Reborn!

Synopsis

A dojo buster has appeared in Namimori. Kyoko and Haru see him in action and are angered at how merciless he is, even against children. They are frustrated that they aren't able to do anything. Reborn tells them that Tsuna will defeat the dojo buster for them. Naturally, Tsuna doesn't want to. But when he sees Kyoko's sad expression, he is unable to refuse and begins training. The name of the technique that Reborn is trying to teach Tsuna is called the "Absolute Evil Fist." But for some reason, Reborn won't tell him what kind of fighting style it is.

(Source: Wikipedia)