666 Satan

After her father dies, Ruby Crescent follows in his footsteps and becomes a treasure hunter. She searches for O-Parts, magical objects buried underground that grant superhuman powers and can be used only by O-Part Tacticians, or O.P.T.s, who descend from angels or devils. Ruby meets Jio, a mysterious boy with a lonely past who wants to conquer the world. Though he is hostile at first, he joins her as a bodyguard. When an O.P.T. claiming to be Satan attacks Ruby, Jio reveals his own power—and that he is the real Satan. The two travel together, hoping to uncover their pasts.