Otherworld definitely is an awesome song in context, you can't deny that
But yeah the game doesn't really have an ultimate villain... Seymour is easily the most evil/villanous guy, but like what was said, he basically just got in your way. Yu Yevon was the final boss, but he wasn't really evil, he wasn't even conscious. Yunalesca wasn't evil either, just cynical.
Honestly that's probably one of my favorite things about the game. The triumph at the game is that you kill Sin forever, the game-long antagonist... but that also means you kill all the fayth. That's probably why the final encounter was on a double-edged sword.
also X-2 isn't canon
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_foolmo_
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