There is a whole complicated compatibility thing between Tidus and the other characters. It involves how often they fight together, how often they heal and use Items on eachother, the beating of a butterflys wings in south america, etc. It would take waaaaayyy too much time and effort to determine and explain the entire system, but if you basically have characters fight together then you should be able to change the compatibility fairly easily. I think gamefaqs.com has a more complete explanation if you really want one. I think the thing in FF7 had more to do with dialogue choices and such, while FFX has more to do with combat.