I think the obvious choice is Ebenezer. At some point the hero has to lose his mentor, and in this case Eb was not only Harry's role model, he's the guy who gave Harry political cover from the Merlin and gave Harry insight into the workings of the Senior Council. Any knowledge Harry has picked up from Luccio, Ramirez, the Gatekeeper and others has been in passing, it's not been in depth. If Harry's status as the Winter Knight; make that Mab's chosen Knight, now gives him a measure of protection from the machinations of some members of the Senior Council; because no one with a brain wants to cross Mab, then Eb's plot armor; if such a thing exists or existed, is gone.
On the other hand Jim might want to surprise us and kill off someone totally unexpected. Their death might even be what Harry has to investigate.
An alternative is the leader of a major supernatural power dies and threatens to unbalance the supernatural world just when peace talks are getting started, whether Harry has to investigate that death or not. So I can see Lord Raith taking the dirt nap in this book. Could Lara and House Raith maintain their hold on power? What would this mean for Thomas and Justine? The answer to the first question is yes. Lara would probably let someone from one of the other Houses kill her father just so she could show everyone she has learned Lord Raith's kiss of death trick.
So let me state that in Mirror Mirror there were no peace talks so whoever dies at that time is alive in the Alt-Harry's world. Maybe Lord Raith is still the actual ruler of House Raith and not a figurehead, but more interesting would be Harry getting to talk to Ebenezer one last time. Well, after Eb stops trying to kill Harry or bring him before the Council.