In this universe, everyone receives a guardian. It's either a demon, an angel, or a mix of both (which is rare tho). They are meant to protect someone until they turn 35. Some like that idea, others don't

Leon was one of those who hated that idea. Why would he need a guardian? Too bad that you get your guardian at 15. And he turns 15 today! Wow..

Leon wakes up and sees a black aura, that means he gets a demon assigned to take care of him. The unknown demon speaks up: