While I kind of hoped it was a new character, it's obviously someone we and Buffy both know. The mask wouldn't be necessary for a new villain, and his appearances and dialogue have suggested an intimate knowledge of Buffy's psyche.
We know it's not Riley, because they've been seen together.
Angel or Spike would piss off WAY too many people, plus I'm pretty sure their licenses at IDW prevent any kind of in-continuity appearances at Dark Horse.
Unless time travel is involved, it's no one we've seen on-panel with Buffy.