Originally Posted by Ouch!
The Final Fantasy characters in Kingdom Hearts have absolutely nothing to do with their counterparts from the Final Fantasy games. They have completely different background stories, in most cases a new wardrobe, and are grouped differently. Why else would Yuffie and Aerith be with Leon? Or why would Selphie be with Tidus and Wakka?
Simply put, they were put in there to be familiar faces that would draw Final Fantasy players into the game. If you think about it, half of the Kingdom Hearts fans would have never even given the game a chance if there weren't FF cameos in it.
Kingdom Hearts is not a Final Fantasy game and so the stories don't fit in with each other. Just completely disregard the fact that the Final Fantasy cameos have their own games and pretend they're original characters. It'll make things so much easier.