Beowulf would be considered the protagonist because he is, for one, the main character in the story. He is the hero whose job it is to defeat the evil and make the world safe for his people, sacrificing himself for it.
Round Character
King Hrothgar should be considered the round character because at the very beginning of the epic, he was a partying, honorable king of the Danes and a viking but one monster turned him into a coward. He ran from Herot and only came back when it was safe enough for him.
Flat Character
Beowulf is also the flat character because throughout the epic, he never once dramatically changes. He is always a strong, righteous, showoff viking that mainly cares about his own fame. He always goes into battle wanting to show off his skills and doesn't ever tell his men to help or intervene. Even when he is dying, he doesn't care, he just tells Wiglaf to take the dragons gold and build a tower for his funeral so that everyone know who he was and where he lies.