I have a theory about Ultimecia... (Major Spoilers)

[Felixrush]

Interesting theory, but I disagree, for a start, you claim that Squall's ring was the summon itself, but I beg to differ, if squall had this GF then why did he not use it during the game?

Secondly I would like to point out that Ultimercia is from the distant future, the game itself states this.

Thirdly, when Squall and Co go to Trabia, didn't they agree to keep a diary or some such? coupled with the fact that they are all seen through the camcorder at the final scene, Squall and Rinoa are together... hell, Ultimercia could even be a descendant of Squall and Rinoa's progeny (now waits for Seraphy to pounce on the idea).

Fourthly, why would Rinoa a girl who claims she feels like she gets left behind when the others have a focus, suddenly completely change from being kind and gentle to being ruthless and merciless, even if the Griever had wiped her memories out (which incidentally would also have wiped out the infected memories from Adell's childhood) she would still have the same personality as proven by the fact that Squall and Seifer are always at each others throats.

And finally the nail in the coffin for me is this: if Rinoa is Ultimercia and she lost her memories because she had Griever junctioned all this time, then why did she announce that she was going to junction Griever "unto herself" half way through the fight? wouldn't it make more sense to have this creature junctioned all the time so she can destory all the "kursed SeeD's".

It could be for the same reason as Solom Ring. He may not have had the right materials and he may not have realized it was a summon. Regardless, it had to be a summon if it saved Rinoa's life in the spacesuit. Why else would they put emphasis on it during that scene?

Rinoa obviously saw Adel's life when junctioned to her. There would be no other reason to mention such a statement. Squall and Seifer knew Garden and had grown up there a majority of their life, in this instance, Rinoa would lose her earliest memories (those of her life in Galbadia) and then she could only have Adel's memories to draw from, which she gained at 17. While that may not make a lot of sense, you have to recognize that GF's seem to specifically target a persons earliest memories (one of the reasons for the Orphanage scene) so Adel's memories wouldn't have been her earliest. It's obvious she saw Adel's memories, perhaps in a similar fashion that the party saw Laguna's (the only example we can really draw from), and I believe she did comment that they scared her.