One more thing, doesn't Ulti change her speech in one of the beginning lines (just before you fight her last form)? I think she says something like "Time...it will be compressed (Notice a "c" and not a "k")." It's subtle, but I always thought it suggested a difference in persona between that form and her normal self. And (Now I'm really getting off topic) I'm not even sure that thing really is Ulti, to tell the truth (the normal Ultimecia is on the bottom). Just thought I'd bring it up.
Ulti is possessed all the way and is free of possession on her final form.
Our heros donīt realize it and keeps attacking.
Ulti feels threatened and attacks back.