Back in Turn Coat, Harry realized that Peabody had a partner with him on Demonreach Island, someone who was never identified. Harry didn't even tell Ebenezer he knew this for a fact. Ebenezer thought it was just speculation on Harry's part. So, we know it can't be Ebenezer, LTW, Ancient Mai or any of the older wardens who were with them. As possible suspects who are at the talks that leaves Martha Liberty, Cristos and the younger wardens.
We know next to nothing about Martha Liberty so there's nothing that really points to her. She once said of Harry to Ebenezer, "You know what he was meant to be?" That's such a vague comment it doesn't mean anything in this context. Cristos looks more like a dupe than a serious player, so unless he's just playing stupid I don't think it's him. More important, Ebenezer warned Harry that he is likely to be betrayed by someone he trusts and that sure isn't Gregori Cristos. Of course, it might be argued that Ebenezer unwittingly betrayed Harry by the way he acted on the dock, but he wasn't the person who was with Peabody. (Also, it would kill my WAG; make that educated guess, so it totally can't be Eb.) That leaves the younger wardens, Harry's closest friends on the White Council and the members who actually look up to Harry. (or at least they used to)
Now it's possible that in Battle Ground some White Council reinforcements might arrive, but I can't think of any of them that Harry particularly trusts. I'm guessing that Rashid will remain at the Outer Gates with Mab and Harry doesn't trust the Merlin or Ancient Mai; in fact, Harry would be looking for a sucker punch of some kind from Arthur Langtry. I know I left out Anastasia Luccio, but after one of the wardens (I think it was Chandler) said that Luccio had ordered them to keep an eye on Harry, I don't think Harry would be too trusting of her either. Plus, the mind whammy she had been under would always be in the back of Harry's mind.
So, if the second traitor is at the Peace Talks I'm guessing it is one of the young wardens. The natural suspect is Carlos Ramirez. He's the one we know best and the one that Harry probably trusts, or trusted, the most. Actually, Harry might be angry with Ramirez and company, but he probably still believes that they're all good guys, even if they don't particularly trust Harry at the moment. Carlos told Harry that he voted to retain him in the White Council, which is exactly what someone who wanted to keep or regain Harry's trust would say. We also know that Carlos is suffering, I'll just call it PTSD, after his near deadly encounter with Molly. However, that incident occurred long after the events on Demonreach. It's not something we can use to tie him to Peabody.
So my guess is that Carlos Ramirez is a red herring and the real traitor is Warden Chandler. He's the warden who went most out of his way to play good cop when the wardens stopped Harry outside the Raith compound. Of the younger wardens, he's the one who is most trusted by the Senior Council and probably the one most trusted by Captain Luccio, because she's his boss and probably the person who assigned him to guard duty outside the Edinburgh complex. A traitor wouldn't want to be working a territory in the field (unless it's a key area) if they could avoid it, they would do everything they could to put themselves near the seat of power and that fits Chandler, perfectly. Finally, Chandler is also the warden who didn't get himself locked up during the events in Changes. Exactly what a smart spy would do.
I'm discounting Warden's Wild Bill and Yoshimo because we don't know very much about them. We've seen Harry talk to Wild Bill on the phone and I think Yoshimo made an appearance in Dead Beat, and both wardens were in one of the short stories. (War Cry, I believe.) We haven't seen either of them build up any degree of trust with Harry. Now I'm going to have to go back and reread every conversation Harry has had with Chandler. If there are any clues they're going to be really minor ones, but I won't be surprised if there aren't any. Anyway that's my guess, what do you think?