I have been thinking this just now:
UFO brought us our Undefined Fantastic Girl, Nue Houjuu, whose main ability is the power to conceal herself, right?
10D brought us her Tanuki Friend, Mamizou Futatsuiwa, whose main ability is the conceal the appareance of other things, incluiding herself, right?
This, with the title being Hopeless MASQUERADE, and the silouetthe (I think I may typed that wrong) that apparently has masks flying around, maybe another third girl with concealing powers will show up?
Now, what youkais that have abilities similar to Nue or Mamizou exists that haven't been mentioned in Touhou?