I was initially thinking that it would likely be one of the listeners. In WOR Eshonai's mother tells her the song of the Last Legion, that escaped their forms of power, for the freedom of dull form. This reminded me of how Marsh was controlled by Ruin. I don't think that Brandon would repeat the same plot element by making one of the listeners the champion. It must be one of the human cast members, and I think Elhokar is a good, bet. He sees faces in the mirror and is being haunted by something, I thought initially cryptics, but why would we see another Lightweaver? We haven't really seen two members of the same order yet (except for Ym and Renarin), so maybe these were Odium or one of the Unmade. It is definitely a plot point left up in the air.