Sephiroth

 "Rengoku. I do not know what it is that draws people to him, or causes others to flee in horror. The power he possesses is far greater than I ever though possible. At the very least, his existence makes mine far more entertaining.' 

Sephiroth died during his last battle with Cloud Strife during the war between Chaos and Cosmos, but was resurrected thanks to Rengoku and Aegis' efforts, freeing him along with select others from the endless cycle of combat the two deities forced them to endure. Upon realizing that his loyalty to Chaos only left him on the losing end, he chose to serve Rengoku's purposes, claiming that he finds his existence interesting and wishes to learn more about the power he possesses and what he plans to do with it.