In a sense the Id,is his own entity, lets not forget back in Fool Moon I think it was when we first meet him, he is a very different Harry from the one we know. I wouldn't be shocked if that is the Harry we meet in Mirror Mirror..
I have an entire theory that DuMorne did
not fail in creating a thrall out of Harry, and to cover the corrupted "real" Harry he created a fake overlay personality- which has now been in the driver's seat longer than the "real" Harry and is its own being, and the main character of the Dresden files.
idHarry would then be "original" Harry. Similar, interested in the survival of thrall-Harry, but not quite the same being. It would explain why Harry hasn't backslid as a warlock- the backlash affected real Harry, not constructed Harry. It'd go into a lot of the philosophy of mind fun stuff, and put a whole different spin on Uriel's interactions with Harry.
I'm not firmly attached to it, but Butcher has left himself room to go that way. It possibly, in a less interesting way, explains some of his incorruptibility by beings like Lasciel- he's actually bound in terms of some of the choices he *can* make- and it would explain Bob's explanation about how a fine thrall could function theoretically forever without knowing it's a thrall.