Griever

Griever is Squall's interpretation of the ultimate Guardian Force. During the final battle, Ultimecia draws Griever from Squall's mind, bringing Squall's perceptions of the being into existence to fight the party. This is made more explicit in the Japanese version where Ultimecia claims to summon the one entity Squall views as the most powerful whose power would increase the stronger Squall envisions it, meaning this manifestation of Griever is powerful purely because of his importance as a symbol in Squall's mind. It could even be seen as Squall needing to overcome a part of himself to win the battle.