In beast form her head flips upside down causing her hair to fall down, revealing a second mouth underneath. Her horns are revealed to be fangs sticking out of this hidden mouth. Her bow (which are actually her arms/hands tightly closed) extend outwards and become big and hairy ape-like arms. Finally, her entire body grows 3-4 times in size and the face distorts with a new pig like nose.
Otherwise...Midi is a good kid, despite looking like that.