The Human Devil - Turns out all humans are devils, but they dominated earth so much that they became the "standard" and don't see themselves as devils.

Just throwing out the idea, I haven't read the manga, so no spoilers. Would make for an interesting higher concept if it ever got explored.

And then there's the whole issue: If there's devils, is there more supernatural stuff going on? Is there an afterlife? So many questions that will probably never be answered :>