Extra thing I wanted to add as a possible twist to the second point: while it's generally accepted that Zero and Yuuki were killed and put into a coma respectively by a demon, and the scene of the attack certainly looked like the scene of a demon attack, part of the mystery around it might be that no demon has stepped up to take credit for it, which - given AU Zero and Yuuki's prominent position as leaders of the Amabilis defense force - seems pretty weird.
Re: ABOUT HUMANS