Ultimicia would actually be a better fit and much less random than Kefka and Exdeath. The whole story since Gaia appeared has been littered with hints of Ultimicia, the most notable Gaia's 'possession' and the idol of Darkness. Also, at one point Gaia brings a straight up Ultimicia quote from the last phase of the Final Battle.

Granted, those hints are only noticeable if you played FF VIII.