I'm pretty sure it's representing Squall, because "Lions" are meant to represent him. It is stil Griever's theme, but it is created to refer to Squal as Griever is suposedly meant to be a part of Squall (I do not believe the S=G theory, mostly because it's part of the R=U theory) I agree that "The Oath" sounds like it could be Squall's Theme.
"Rivals" plays during the interigation scene in D-District Prison - that scene fetures Seifer quite frequently, I'll check if there are any more.