Ultimecia. She had absolutely no backstory like most villains did and she was only there because the game needed a villain. We found out that she's a sorceress trying to compress time; so what? At least Zeromus being evil was explained by the fact that FuSoYa had sealed him in the moon and he wanted revenge. Sephiroth's back story was explained by Cloud's flashback when he discovered about Jenova. And Kuja's backstory was explained by Garland that he was a creation like Zidane. You find out much more about them rather than just the obvious facts that they are evil, unlike Ultimecia. And you never really got meet her until the end of the game.

aNd aT tHe EnD, sHe taLkEd lIkE tHiS aNd kOuLdN't gEt hEr c'S rIghT. The way she talked was not really evil, funny, or whatever it was supposed to be. Just plain odd (and odd things are not what I would expect villains to say).

By the way, aren't Seymour and Shuyin the villains of FFX and X-2, not Sin and Vegnagun?