Cloud: Higher HP
Aeris: Allies near death
Barret: Higher AP materia
Vincent: More enemies killed
Tifa: Limit gauge
But I haven’t figured out the rest, I would really like to know if anyone has any ideas as to what strengthen the other final weapons O_o
Cid and I believe Cait Sith also: Same as Cloud
Yuffie: Enemy’s level is lower than Yuffie’s level (I think this is it, I know it has something to do with the enemies levels. I always seem to get 9999 on hers, so it’s a good powerup.)
And I remember Yuffies, which I left out, it’s the enemies level, the higher the enemies level, the stronger it gets (so basically, it does LESS damage on weaker enemies O_o)
And Cid’s is based around his max MP, not HP. You’ve got the rest of them right. 🙂