Cloud- HP total
Barret- amount of AP on his materias
Tifa- Limit guage (fuller it is, the more power she has)
Cid- MP total
Red XIII- MP total (i think)
Vincent- amount of enemies killed with ultimate weapon
Yuffie- not quite sure, i thought it was the difference between her level and the level of the enemy she's fighting?
Cait Sith- HP total

if im wrong.. sorry!