I’m on episode 15 and I’m thinking it’s the empress.
I don't think so, the way the masked man carries himself implies it's a man.. I could be wrong as well. I believe that the Empress could not have left the palace (to see Xia JingShi at his camp) without someone noticing her absence.
I'm in the middle of episode 19...... but I have this feeling and I hope I'm wrong....that the masked man is Feng Suige's best friend, Murong Yao... I keep going back to Yixiao's words and how Suige has been constantly betrayed by the people closest to him.