Reborn! (2006)

Hitman Reborn is the story of Tsunayoshi Sawada, a spineless teenager nicknamed 'No Good Tsuna.' One day a baby hitman shows up & tells Tsuna that he's to become the new boss of a powerful international mafia family. Over time he whips No Good Tsuna into shape to take on the evil Byakuran who wishes to take over the world.