I didn't like this game, but I liked Tidus. I thought his development was well-crafted, and he made the transition from whiny brat to whatever you want to call him quite smoothly. Tidus really seems to be the only character who develops at all. Wakka might be the next best thing, but he just suddenly isn't a biggot. Yuna always wants to help people the best way she can; the method just changes.
I'm also probably the only person who has played this game who couldn't stand Auron, likely because a good deal of this game relies on people not saying really obvious things the whole time for no particularly strong reason (sure, there are reasons, but ones I'll buy?), and he's a huge part of that.
I also found myself disappointed by how predictable everything was and how slow all the characters seemed to be. After Square went through the trouble of setting up a detailed world and a complex history for it, they couldn't carry it all the way through.
The odd thing is that I still played it twice. So even if I was really disappointed by the plot, couldn't stand the characters, and found the aesthetics laughable, I have to say that there's something that makes this game good. There are a lot of reasons it doesn't work for me, but I play games because it's fun, and a complex plot and characters are really just bonuses. Game is good. Other elements, not so much.