666 Satan

In the not too distant future, mankind battles over O-Parts, relics from an ancient civilization. It takes a special kind of person to unlock the full potential of these remnants from the past, and a unique breed of humans known as O.P.T.s (or O-Parts Tacticians) become a force to be reckoned with. Unfortunately, O-Parts can be used for good or evil purposes.

Jio is a young boy with a tragic past who only trusts one thing in the world: money. Little does he suspect that he is actually a very powerful O.P.T., and inside him sleeps a demon of incredible ferocity. He meets up with a girl named Ruby, and together they decide to embark on a dangerous quest to discover as many O-Parts as they can. Will Jio help Ruby realize her dream of becoming a world famous treasure hunter? More importantly, will Ruby help Jio realize his dream of--world domination?!

(Source: Viz Media)

Included one-shots:
Volume 6: Trigger