Beelzebub
Beelzebub is a Japanese manga written and illustrated by Ryûhei Tamura appears in Shueisha's Weekly Shônen Jump magazine.
It's about a juvenile delinquent, Oga Tatsumi, who is a first year at Ishiyama High, a school for delinquents such as him.
One day after a fight, Oga pulls Alaindelon out of the river. Alaindelon splits in two to reveal Kaiser de Emperana Beelzebub IV[aka Baby-BL], the Demon King.
With the help of Hildagarde, a demon maid who was tasked to help raise Baby-BL, and his best[ and only] friend, Furuichi Takayuki[who Oga calls Idiot Furuichi], Oga has to fulfill the demon contract[which takes form of a tattoo on the back of his hand].
I've read the first 10 chapters of Beelzebub and I really, really like it. To learn more about Beelzebub, go here and to read the first 10 chapters in English, go here