Actually, Yu Yevon's that weak normally, and he uses the aeons as cover, to avoid the party's attacks. He confronts you when all his cover is blown.

In FFX-2, (SPOILER)After YRP defeat Shuyin, it shows a scene where the Fayth (kid in a purple hood) talks to Yuna, saying that they can "bring him back" if she wants. You are given two choices, to accept or decline the offer.