I found that yuna held on really well for someone with a burden of having to save the world.I would agree that a lot of people would break from stress of such a deal of having to sacrifice yourself for the world.
Jecht:I got the impression that at first, he was just a thug and a troublemaker, and later on when he joined braska's team, he realized that what he was fighting for was bigger than himself.He seemed different at the end for that reason.
Rikku:For someone who's city was blown up, and may friends of hers were killed, it is interesting.She's not my favorite character, but she is alright i guess.
Auron:In some ways i get what square was doing, but i never cared for him much.He has an interesting background story though, and is way better than kimahri and lulu.
Lulu:I found her to be of less use to the story than auron, and auron is a big deal character.
Wakka:I found a lot of his dialogue pointless, and sometimes he seems a bit too happy go lucky in horrible events.
Tidus:In some ways, i get him as well.He is weak and a mama's boy.Maybe a bit even of a goody two shoes and a show off.

Overall:I think FFX has my least favorite roster of characters.