Reborn

Reborn is the infant home tutor of Tsunayoshi, Dino's former tutor and the world's strongest hitman.

He remains fairly calm through any situation and always putting a straight face, even in his "fights" against Colonnello. He also is rather sadistic and likes to occasionally tease his students, much to both Dino and Tsunayoshi's dismay. Bianchi is his ex-girlfriend.

Spoiler

Reborn's real name is Renato Sinclair. He's one of the seven Arcobaleno. Reborn is actually an adult, but has been turned into an infant due to the Arcobaleno Curse.

Reborn! Wiki.]