A good point.  So, that would probably only be the British (sounding) prisoner that Thomas might talk to, but only if that prisoner wanted to speak to Thomas. 

What I find striking about that encounter, is the three-eye addict; who would have been a total newbie to the world of magic compared to a wizard, recognized the name of the Walker, as if it was written in bold letters on Harry.

I had been presuming a sort of pre-programmed simulacrum, a miniature "spirit of knowledge" along the lines of Bob (or Lash & Bonea (and likely the Archive)) that Margaret had constructed for Harry before her death; albeit a vastly-lesser one.
But for this thing to be tracking all sorts of extra info -- Thomas, and Thomas' girlfriend -- happening out in the world, it'd need to be considerably more robust, not just pre-programming.  Able to conduct reconnaissance, to learn, to adapt... not really "lesser," in fact quite robust.

If Listen is still alive, which seems doubtful after Harry buried him under a wall of dirt, he might be a candidate, but I'm just not feeling it.

Jim stated that Drakul thought that Kemmler's behavior was amusing, something that was funny.  While that gives us a clue just how twisted Drakul is, I think it also tells us that he didn't rate Kemmler that highly.  Kemmler was a very dangerous person, but even though things like setting up World War One was supposedly one of his projects, I suspect the mad necromancer was somewhat random in his depravity.  Beings like Nemesis and Drakul rank higher on the bad news scale because they have a plan.       

Also, I keep forgetting to answer the titular question, every time I come into this thread.
I keep getting side-tracked by all y'all's interesting ideas!

I think the most likely partner for Summer is the Svartalfs.  Wizards and the Fae don’t really trust one another so I don’t think the White Council would make a good candidate, though I’ve been wrong before.  The Swartalfs probably suspect that Lara Raith had something to do with Thomas Raith’s escape from the dungeon in what was formerly Marcone’s castle and of course they are right about that.  The Svartalfs still have a score to settle with Thomas and they may suspect that Harry has found a magical way to shield Thomas.  The Swaretalfs are normally a very insular people but circumstances might make them rethink their normal policy of remaining aloof.