I don't really understand the backstory. He was adopted into a rich family, but then they got a NEW child who became the favorite, and also some other kid with a forehead tattoo was there for no reason? but B.J. killed the forehead kid and the new kid, and then the rich family (who were actually mafia) started pursuing him.
Like, that's a LOT to take in. Then crazy headbutt attacks! This manga has some wild swings in tone lol