Reborn! - Akira Amano

Tsuna, a timid junior high student, is a failure at school, sports, and his social life. But everything changes for Tsuna when Reborn, a fedora-wearing, gun-toting toddler who claims to be an Italian hit man, shows up at his house. Reborn was sent to Japan to groom Tsuna for his future life as a big mafia boss! Reborn's gun has quite an unusual effect: When Reborn shoots people in the head, they die but are then 'reborn' with temporary supernatural powers that allow them to fulfil their dying wish. Reborn and Tsuna make an unlikely pair, but every day they're together, Tsuna gets reborn into a new (and better) guy!