Since Douman is apparently going to be an Alter Ego whenever we finally summon him according to those CMs from before Lostbelt started, I have a crazy theory: the person he's an Ego of is Abe no Seimei. He did initially claim to be Seimei in Shimousa. I'm betting it will be something like "Seimei's great rival was a split personality and not a separate person" or something.
Or maybe I'm reading way too into it.