I'm probably confused with your use of the word ghost Serapy. I take it by ghost you're refering to the Fayth.
Yu Yevon was responsible for the summoning of Dream Zanarkand, and the creation of Sin by magic. YY never drained souls from anyone. Pyreflies are likely the soul of one's soul in the mythos, but the pyreflies (or souls if you prefer) YY used to build Sin were already dissociated from people (ie, the souls were already dead and just weren't sent to the Farplane).