I believe it's Sephiroth... I'm probably wrong but here we go.

My fate is uncertain, <-- after being thrown into the reactor by Cloud we don't know excatly what happened to him.
Its based on a whim.
Ego and life contest. <-- Ego, Superego and Id theroy. Ego is the Self moderating the two polar oppisits of Superego and Id. The place where "Self and Life contest" is the lifestream. Sephiroth was thrown into the Lifestream

We have met before, <-- Cloud and Sephiroth met previously
Or we will meet again, <--- and are fated to meet again
Aside the scarlet unknown. <-- Aside the unknown scarlet thing that holds Holy.

And if that doesn't work I'm fresh out of ideas.