Yeah, I posted, then re-read what you said and saw that it was basically what I wrote. But to re-iterate, what I said was that the pictures of the Six Heroes looked like it was Hazama after Terumi was revealing himself at the True End, so he was either possessing Hazama BACK THEN (meaning Hazama is old as hell) or it was just Terumi all along.
But... who else would it be? It would be kind of random to just introduce a new character he was possessing back then. We're not really sure what the deal with him being a ghost is either, since in some routes he is a ghost, and in others it seems like he's Hazama.
Actually, wait. In Rachel's route, don't we see him as a ghost AND as Hazama? In the beginning we see him as a ghost, and then later we see Hazama as Terumi when the world was about to end.