The evidence is largely circumstantial, but given that he have never seen his persona, I think there are only 4 options:

1 - He doesn't have a persona, he's just controlling Trollocs. Boring. Plus I'm 100% sure he's masqueraded as Rand to control Masema, and probably other people too.

2 - Roederan. Has some potential as a theory but the guy has no standing army, he just recruited Mercs to fight for him. Murandy would be a great launching point for a Trolloc attach though.

3 - The lands to the East (I forget what it's called). Doubt it, since we've barely heard anything from there.

4 - The Sea Folk. They have the greatest mobility of anyone in Randland aside from Travellers, and the ability to attach almost everywhere else while TG is fought in the Borderlands. They have a huge number of channellers, all of whom can freely use the power as a weapon and have experience doing so. This makes them an extremely useful force. And they take their bargains extremely seriously - all Demandred had to do was pose as Rand, make a dumb bargain, then break it (actually, Rand has already broken his bargain, and Brandon noted that there would be consequences).