Cerberus Manga

From Easy Going Scans:

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!

* Cerberus, in ancient Greek mythology, was a hell hound that guarded the gates of the Underworld preventing those who have crossed the river Styx from ever escaping.