Well, XS Episode I is way better than XS Episode II. Then again, I liked the second episode's battle system better. I think both games were really nice. I like the story very much. One could only guess what would happen next.
As for chaos being an angel, well, I have no idea. Sure he has all these official arts with six wings but... Well, I have no idea what he is. But I'm sure he's not human.
