Out of curiosity, could the unlockable characters possibly be god and satan? I noticed that they juxtapose each other, and the likes listed under "powerful demon" could be a reference to the seven deadly sins. Well, I don't know.

EDIT: Oh I just noticed scarlet's comment:

"Also, I already figured out one of the unlockable characters. The English title of the song is right below the other version of the song."

^What? Where? I don't see it D: