Flat and static characters are somewhat necessary in texts. While their lack of description provides readers with little to no information about the character, many are stock characters that readers immediately recognize (even with the limited description).
In regards to the character of Grendel in Beowulf, one could make very different arguments regarding him being a flat character. One cannot argue his existence as a static character though (he never changes--he lives as an antagonist and God-hater and dies as an antagonist and God-hater). While one could argue the flatness of his character based upon his or her belief that he is one dimensional (lacking depth; superficial), another person could argue that he is not superficial or lacking of depth.
Argument For Grendel Being a Flat Character
By definition, a flat character is one which is not defined or described fully yet fulfills a narrative purpose. When applying this definition, Grendel is most certainly a flat character. He is not really defined literally (for some), and his character is used to prove Beowulf's heroic nature.
Argument Against Grendel Being a Flat Character
In chapter one of the text, Grendel's background is clearly defined. The ancestor of Cain, he has been banished by God to live in darkness. The description of Grendel's character is given as one which describes him as "full of envy and anger," tormented by the praise of God heard at Heorot. His exile forced him to dwell in darkness, surrounded by other monsters.
In chapter eleven, the utter strength of Grendel is described. He is able to gain access to Heorot, which is chained and bolted with steel, through simply hitting the doors with his fists. Grendel can devour a full grown man easily. This said, Grendel also shows his surprise when Beowulf grabs him.
During their battle, Beowulf and Grendel all but destroy the mead hall.
Given these descriptions, one could consider Grendel to be a round character (the opposite of a flat character). A round character is defined as a character whose personality, background, motives, and other features are fully delineated by the author.According to this definition, Grendel is a round character. His background is given (his ancestry to Cain and exile from God's light), as is his personality (envious and angry) and motives (taking out his anger on the people of Heorot given his exile).
When examining flat characters we must ask the question "Has this character transformed from the beginning of the story to the end?"
In Grendel's case, I would say that he is the very defenition of a "flat" or "static" character. In fact, I would say that Grendel is not really a character at all, per se. He is a mechanism, a foil, to allow Beowulf to demonstrate his strength, heroism, and utter faith in God.
Grendel is dead at the end, but that's not the kind of transformation we are looking for. He follows the same pattern throughout the story: attack Herot. That's pretty much all he does. His motive for attacking is his distaste for the Chrisitan hymns being sung. At the end, he still hates the songs, the singers, and especially Beowulf. He runs away from Beowulf, after getting his arm torn off, but this just backs up his anamialistic behavior and isn't indicative of any real change.
The interesting part of Grendel is that he represents paganism, and thus Satanic evil, in an early Chrisitan setting. Hrothgar's people are teetering between the salvation of Christianity (surely how the unseen monk narrator/translator saw it) and a past world of dark pagan sufferring.