I would be Yomen.
I could tell you why, but that would be a spoiler.
There's only a couple things we know about Yomen at this point. He was an Obligator from Urteau that was opposed to Lord Cett and invaded Fadrex when Cett invaded Luthadel. At this point 'The Citizen' took over Urteau, assuming that Yomen is not actually Quellion. It's possible that Yomen is the third voice in the conversation, the one that says 'Until that bastard Venture took control'.
Anyway, I've been curious about Obligators for some time, because of a quote paragraph or two that seem straightforward but could have a different meaning.
“My . . . obligators . . .” the Lord Ruler whispered.
“Your obligators will forget you,” Marsh said. “I will see to that. The other Inquisitors are dead, slain by my own hand. Yet, the gathered prelans saw you transfer power to the Canton of Inquisition. I am the only Inquisitor left in Luthadel. I rule your church now.”
“No . . .” the Lord Ruler whispered.
......
The Lord Ruler cast his eyes down at the crowd, and the final realization of his failure seemed to hit him. He looked back up at the ring of people who had defeated him.
“You don’t understand,” he wheezed. “You don’t know what I do for mankind. I was your god, even if you couldn’t see it. By killing me, you have doomed yourselves. . . .”
Look at it in this light. The Lord Ruler was trying to say something important about the Obligators but Marsh, who may have been controlled by Ruin at this point, interrupts him. I seriously doubt that TLR cared about whether he would be remembered anymore. Then he looks down at the crowd, or at least in their general area. It could also be in the general area of the Canton of Orthodoxy Headquarters. Hmmm? Hmmmmmm?