Hannya appears to be a Shy Boy slightly facing sideways and wearing a mask to hide his real face. Has a slow attack rate, but does massive damage and weakens your cats from a range of 800. Try using cats that can tank a hit and fight back, like Divine Kuu or Pai-Pai. A well-timed suicide bomber like Awakened Bahamut or Tropical Kalisa can deal insane damage as well.
Unlike Shy Boy, he is not considered a Floating enemy as he isn't actually floating.
Fish Cat, Titan Cat and others are useful against him as they do not die in 1 hit
Wore a creepy mask as a joke, transformed into a flesh-eating demon. That can happen. Breathes pure corruption born of vengeful hate. The normal Face isn't so bad anymore, eh?