So like a long, loooong time ago during my first playthrough I didn't use Aeris all that much. And in every playthrough since then, I never really used her cause I didn't see any point in it. But not that there's an achievement for getting her Level 4 limit, I've been grinding her up and she has some of the best. She can fill up everyone else's limit, has free Esuna, a free heal, can turn everyone invincible. Wish I knew this earlier.