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Maybe it's interesting to look into this, but yeah no Rinoa is not Ultimecia. She may have been a sorceress, she was used and possessed by Ultimecia before, but she was never Ultimecia herself. Ultimecia was just another sorceress from a different era.......maybe this theory would have worked if only the people who created this game didn't disprove this theory themselves. If Rinoa was Ultimecia though, it would add a lot of deep emotional plot development into this game.

That's like one of the crappiest theories about VIII. The most convincing once I've seen is how people believe Squall died at the end of Disc 1 and the rest of the game is a hallucination in his last moments (but that was also very crappy too. A little convincing at certain points but crappy.)

None of the theory is backed up by decent in-game evidence, not to mention what Squall said about Ultimecia being a sorceress eras after VIII's present timeline (I think they officially said about 100+ years or so.)

Plus, biggest evidence against it is; if she was Rinoa, WHY THE HELL WOULD SHE LEAVE HERSELF TO DIE IN THE COLD VACUUM OF SPACE ON DISC 3?
If Rinoa wasn't saved by Squall (who's awesome personality went down the pooper at that point), then Rinoa would've died, Ultimecia would've been erased from existence and the game would've been over. Let alone the fact that if 'Mecia was "insane" like part of the theory suggests, she still would've known, I think. Even crazies recognize things (I would know. I'm crazy.) so insanity isn't a valid excuse for not recognizing herself, let alone Squall and co. when they break into her house, murder all her pets and then bust into her room for a psycho lecture.

I think a better theory would be about Griever.
I know that it's been said that Ultimecia's power can extend into the reaches of the minds of others (GF Griever and the Apocalypse magic) but I don't it's THAT simple, even if she's a time/space absorbing sorceress. A friend of mine and I are a part of a Final Fantasy VIII project on Second Life and we tend to discuss a lot of points in the games as inspiration for the project. One such thing we discussed was the film that started Seifer's desire to become a Sorceress' Knight (the one Laguna starred in with the little mini-game against a Ruby Dragon) which somehow got twisted into an origin story for Ifrit and the Fire Cavern. It was pretty creative and it sounded very feasible.
And that got me thinking, what if something like that happened to Squall somewhere along the line and HE became that Guardian Force?
I know it's not much of a theory as it is an idea, but still, I find that a bit more believable than Ultimecia simply mind-raping Squall and using that against him.

To fully refute that first part, Squall was only shanked on his right side and in an area that, albeit barely, missed any major blood vessels. The wound would have bled plenty, but he would have also lived. It would be another thing if he were pierced on the left side where his heart would be.