Maybe not a "villain," but Yu Yevon/Sin is/are definitely the central antagonist. Some people say Seymour but Seymour is such a bad character that I don't really buy that at all. He's never really a threat; just some crazy guy who keeps getting in your way.
And as for the last question; wasn't it heavily implied that Tidus comes back at the end of FFX anyways, what with the scene with him floating up to surface of some water? I mean, I only played FFX after X-2 was already out, but what ELSE is that supposed to be?
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You have no time to ponder such questions, as Link has just grabbed a Smash Ball.
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