I don't think it can be an unmet character - otherwise why hide his face.  He is clearly strong enough to play in the big leagues and wouldn't need to hide - unless he was supposed to be dead and isn't OR he is on the White Council as a spy.  I don't think he is currently on the White Council - that was Peabody.  I'm sure there are others there, but unless it is Arthur Langtry I don't see any other SC members possible.  Simon is a big guess, but he was a friend to McCoy and seems likely Peabody is responsible for his death.

That is why I vote for Justin Dumorne.  He has obvious ties to the Outsiders - and he was manipulating Harry since he took him under his wing.  With Kemmler's teachings (via Bob) he could have easily faked his death at Harry's hands.  That would also explain why Cowl doesn't just kill Harry and end it (even though he clearly could).  Justin has had plans for Harry all along - and a wizard that survived the assault on Kemmler probably isn't getting taken out by some apprentice...no matter how innately powerful Harry is.  Kimori = Elaine (she was never out from under Justin's control) - and Elaine's help in previous novels was all her part in leading Harry to the final manipulation.