Replying...
Intro. You were once a Soldier, Sephiroth's partner, and fought alongside him. But because of your immense power, Shinra feared you would become as evil as Sephiroth, so they decided to hunt you down and seal you away, hiding you in a place they thought Sephiroth wouldn't find you. But it didn't work.

Sephiroth.

@Mrs_Night13