During the Jenova fight after her stabbing, Aerith is revived as a zombie and fights against you. If you beat Jenova without killing Aerith, she survives the scene - but she's very fragile, so you're pretty much locked out of using any magic with the All materia, having the counter materia equipped, or summoning, any of which will pretty much OHKO her, and you have to just tank her pretty high magical damage and debuffs.