It's Shuin they're fighting. He's a baddy from a thousand year ago that has a few things to work out with Lenne, the spirit that takes over Yuna's body. Yuna's search for Tidus is like the secondary story line to the main story line. And there's three endings to the game. One you find Tidus, one you don and Yuna is like *oh well* and the last is the BAD ENDING. That's when you do EVERYTHING wrong and you suck.

*grins*