Mystogan is a lone member of the guild.  Laxus had indicated that he knew what Mystogan looked like and what his real name might be, but when Laxus says, "Another," he is interrupted by Mystogan before he could finish.  He doesn't want people to see his face in order to hide his identity, especially from Erza because his face reminded her of Jellal Fernandes.  He seems to be very knowledgeable about the world of Edolas as he instructed Fairy Tail members to perform specific tasks.