Like in most of the final fantasy games i play, i try to make all my characters equal so that, if one dies, i can always switch in another character that is as good as the one that died. But it seems that near the ending i have to end up picking which characters to use since if i don't it would mean even more endless training to make them good enough for the next area. Now in FFX i didn't have this problem as bad as i did in other ff games such as 7. But by the end i stuck with Auron, Tidus and Yuna.
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He helps me in the worst situations of all, and is a tad-bit stronger than Tidus. But like Prophet said, and I quote...
But that cool-ness of him made him become my best fighter. Tidus is used for Steal or other neat abilities in battle, but I make sure he fights the big bosses and such. Lulu is good, but not a main useage I have. Wakka is good, but not very helpful. Kimahri is a strong fighter with Auron, but I try to balance the party. And Rikku... Well, she's good at the throwing bombs and Mixing commands, but I made Tidus learn those, too. ^^; Well, that's my main three.