First of all, Grendel's mother is angry. The statement "there is no wrath like the wrath of an angry mother" fits her well. Of course, if someone killed your son, you would be angry too. Right? She lives in a dark cave-like structure at the bottom of a cold,deep, murky lake. Although the lake is cold, it looks like a fiery flame (resembling Hell). You really have to use your own imagination to decide what she looks like. She is a monster since she is Grendel's mother and he is a monster, but neither are described precisely. We do know they had huge claws.