The displays of griever are very momentous. In most of the events, Rinoa + Squall = Displays of Griever.
You also see Griever after when Squall and Rinoa hugged, and then the game ends. In any game, what we see in the end is important. We saw Griever as the final thing of FF8.
compared to...
-Rinoa asked Squall if she could wear Squall's ring. Suspicious.
-Rinoa looked at the Griever when she was lost in Space, because of what reason? To keep good faith, after that, Squall has found her. Suspicious.
-there are so many more.
Griever is a symbolism, being a symbolism, there is so much to it.
This explains WHY Rinoa became very interested in Squall at the beginning of the game, automatically faniced him, automatically cared about him, regardless of his careless behaviours towards Rinoa. It's like Griever brings a good faith towards Rinoa and Squall.
-Rinoa was insane when she was possessed.
-When she was possessed, Rinoa always knew about the Griever thing, she knew it was very important to Squall.
Rinoa is Ultimecia at some point. When you fight against Ultimecia (remember Rinoa is insane when she is Ultimecia) and she manifested Griever out of Squall, all because of Rinoa's insanity. If it was Ultimecia, not Rinoa, then how did Ultimecia know that Griver exists in the mind of Squall? Unless he was possessed, but no, that didn't happen. Rinoa is the only person who has that knowledge, therefore she was the only one who manifested Griver out of Squall due to her insanity. At times when Ultimecia possessed Rinoa, Ulti did not acknowledge anything about the Griever thing but there are times that Rinoa has acknowledged about Griever and she wasn't possessed, so I doubt Ultimecia has took that 'Griever' knowledge from Rinoa and then manifested Griever.
CTSE.