Cerberus

Cerberus (ケルベロス) is a manga written and illustrated by Fukui Takumi.

Plot
Kei often gets involved in fights, despite of him being weak and always end up losing… One day, while helping a friend to bring some textbooks to an old building of his school, he and his childhood friend, Tomoe, are attacked by a monster. In order to save his friend, Kei will choose to become a Grave Protector!