The protagonist is the leading character of a work of literature, theater, or cinema; the one with a goal. Our main character (and hero) Beowulf is the protagonist of this poem.
flat character
A flat character is one dimensional, serving purpose only to further the plot i.e. has little to no character development. Grendel is the flat character of this work, as his sole purpose is to be defeated by Beowulf, thus furthering the plot.
round character
A round character is a character possessing the depth and complexity of a living person. Beowulf is also a round character in the work as well as the protagonist.