Griever's concept was really innovative. Ultimecia knew she had to find a GF that would be strong enough to take down the heroes, but the GF she had were either too weak or already dead. So she looked in the most unlikely of places: Squall's mind. Inside his mind Ultimecia found the image of a lion as the most powerful being on the planet, and fierce and proud at the same time. Ultimecia loved the idea, and drew that image from Squall's mind, and gave it life into the most brutal GF ever concieved. It shows you how powerful Ultimecia inot just by being able to convert Griever from a thought into a GF, but being able to junction herself to a GF without losing control over her to Griever, as the Junctioner usually controls the junctionee.