Answer:
Grendel is a flat character in Beowulf because he has a one-dimensional personality.
Step-by-step explanation:
In the story Grendel stays a mystery as the story is told from the third person point of view. What makes him seem evil is that he is a descended from Cain. There is less development in the character of Grendel and have been provided with an unclear descriptions.
Grendel is sometimes referred to as a flat character because he does not show a complex range of emotions, he is known as a monster, and doesn't change throughout the story, which makes him seen as a flat character.