I think this is all news to me save for the overlaps with M's history and I think a mention from someone though I don't remember who about a lot of history between Rezo and Xellos. Can I steal the bit about the relationship and the magical runes being put into Xellos (owie!)? I think Val might have that in his history.
That distinction between demon and man is something M and I have talked about and how much we both hated that scene in Evolution-R that implied that Rezo was always evil but pretending to be the loving grandfather because it destroyed the power of Shabranigdu and of Rezo because their was no struggle between them. I'm totally with M on taking nothing involving Rezo from that flashback! I have to say I adore how in character Zel is for his massive mental blocks about Rezo. :P