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I don't think that's an alt Sephiroth. I think that's OG Sephiroth, come back in time, and that he totally played the party. I don't buy that the time ghosts are meant to be a knock at fans that don't want anything to change, because beating them was a bad thing. "Defying fate," in this context, is them veering off the path in which they win. They see the ending in which they've succeeded in saving the world and think they have to avert that.
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