TyrantRaveOmega

It's called "Death and Republic" . You can find it on the Guilty Gear Original Sound Collection: http://guilty-gear.wikia.com/wiki/Guilty_Gear_Original_Sound_Collection

Let's see... In the original XX: Looks like he appears in I-no's path 1 ending, Axl's path 1 ending, and Eddie's path 3 ending. Been a while since I played the original XX story mode. This is a useful reference if you want to no more about the endings: http://www.gamefaqs.com/ps2/562111-guilty-gear-x2/faqs/27246 (massive spoilers) As for AC+... I think he appears in the Anji and I-no stories but doesn't reveal any more surprises like he did in Axl's XX story. Hope this helps!