Round Character one possessing the depth and complexity of a living person
beowulf is round because he started off as being a man who felt he could conquer all eventually that led him to his death. But in the beginning he never used any weapons, since he felt he didn't need them. He eventually started using weapons to defeat the monsters. Beowulf changes a lot through the story, which is why i consider him a round character.
Flat Character one dimensional serving purpose only to further the plot
Grendal is a character that get's killed by beowulf and takes the place of beowulf's second monster to defeat. Grendal has no real big purpose in the story he's whole reason of being in it is to be beaten by beowulf, he has no character development but he does bring out his mother which also leads to beowulf killing her.