It also depends on some of the options you chose in the main game i.e if you flirt alot with lulu in the convosations you have with her it will be her who you like a little bit more, and if you go speak to rikku in the thunder plains when she wants to stay in the travle agency thing then some points go towards her. and if your mean to all of them kimarhi goes with you on the snow mobile and you dont see who throws the ball to tidus.

oh btw blizts ace is tidus's final overdrive. I think you get it by using all the other ones alot.