FFX-3 Chat

I believe Yu Yevon was a normal human who summoned his aeon (Sin) to both combat Bevelle in the war and protect Dream Zanarkand, but Sin's awesome power drained him of most of his humanity and he and Sin soon forgot what their purpose was, instead destroying any trace of civilisation with exaggerated strength. He could have just been a human turned mutant by Sin's influence and old age killed him.

I'm in the belief that Yu Yevon, Seymour and Shuyin are all special kinds of unsent, since they are able to take control of other living bodies and do things in the 'living' world. Maybe they'll expand on that with these three new characters. They're all unsent themselves, after all.