I'm thinking TiM, but I also feel that may be too obvious. I don't know.

I personally don't think the illusive man would communicate through the faces of others, the alliance may know his face but I think that's literally the only thing they have to go on. It just doesn't fit his character to communicate that way, TIM was always seemingly upfront with those he had an interest in, regardless of ulterior motives. I could be wrong though as it is different writers who may have a different interpretation. I'm hoping it's someone more unpredictable, somebody that came along with the arks, hopefully somebody we haven't met yet because most of the characters outside of the crew aren't insanely cool. Other than the turian Spectre, he was pretty cool. Oh and Efra, Evfra? He was also an interesting character.