Grillby is a flame monster, who runs the Grillby's.
Story
Grillby's magic is gone. So his fire is burning his own soul like acid. He was preparing the grill to cook Aliza alive, until Sans and Papyrus saved her.
Appearanace
Grillby is in huge pain. So his face is locked in pernamently agonic expression. His eyes are shown to be just blank white and round. He still wears his glasses and suit. In some pages, color of his fire changes, when feeling strong emotion.