... Near the end of Turncoat, Dresden and McCoy are talking about the fact that Peabody wasn't by himself on the island. Who was the 2nd person? Did we find out and I missed it along the way? Do we think this was Cowl joining him in Chicago? Some other Black Council wizard/entity? ...
The "most sensible" answer, IMO -- and almost certainly, the
wrong one -- is one of the Wardens, as a bodyguard. It'd make excellent tactico-strategic sense to bring one along; and he has ample mental controls in place to do so (he had Luccio commit murder against a senior council member!), so bringing along a "fine thrall" combat-wizard looks like a very good tactical move.
But, of course, that's just a narrative dead end; there's fundamentally no mystery in that, no "payoff" in dropping the hint, little to no
interesting "reveal" later as to who it was.
How about it being Cristos, or some other White Council member (maybe one who's also Black Council)?