Cerberus

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!
Note: 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.
(Source: Easy Going Scans)