Good question. I like Fish.Staplers idea in principle, but i'm a Bastila fanboy, so i'm sort of against that
I also agree that it's unlikely to be Revan, and i would be disappointed if it was. Perhaps the Exile? After all, she probably felt betrayed by the Jedi Council after her exile, and i imagine she felt somewhat bitter after events on Peragus etc., so she would be as likely as anyone to betray the Republic.