Something I read about recently is a theory that Ultimecia is Rinoa in the future. I find this theory quite interesting.
(visit
http://www.balamb.org/ultimecia/ and
http://db.gamefaqs.com/console/psx/file/fi...e_ultimecia.txt if you wanna learn more. The first link is pro the theory and the second is against it)
The fact I think this theory is true. Although, I disagree with the idea of Rinoa getting her sorceress abilities from no where, which is presented in the facts on the Ultimecia fan site above. I believe that after Squall defeats Edea, her body believes she is dying and immediately passes on her powers to Rinoa. Edea miraculously lives.
As far as the idea of the Griever GF goes, I think that Squall is aware of Rinoa's extended life span and at his death still cannot let go of his desire to stay with her. His soul, in a sense, manifests himself into an eternal beast. GF does stand for Guardian Force, hinting at the idea that the creature's will is to protect their keeper.
That's just my belief on the whole thing. What do you guys think?