Grendel is a flat character in Beowulf because he has a one-dimensional personality.
Grendel in the story stays a mystery as the story is told from the third person perspective. What makes him eviler is that we know he descended from Cain. There is less character development of Grendel and have been provided with vague descriptions. It provided he live with his mother in a cave and that he attacks Herot for being noisy. He is illogical in attacking the men and its lineage. Therefore, being completely alien to the human experiences and he does not pop off along the plot, he is considered to be a flat character.