What are the character traits of Grendel's mother in "Beowulf"?
Grendel and his mother in Beowulf are both what critics call "flat" characters. They do not have any great degree of psychological complexity but instead exist purely as antagonists to Beowulf.
Grendel and his mother are descendants of Cain, the firstborn evil son of Adam, who murdered his younger brother out of jealousy. The Beowulf poet portrays them as evil and monstrous. From a modern perspective, we might argue that Grendel was perfectly justified in attacking Hrothgar and his men because they function as invading imperialists who steal his land, but the poet does not actually make that argument. Instead, Grendel is portrayed as hating the Danes out of pure malignancy.
Grendel's mother is even less of a fully realized character than her son. She lives in a cave under a lake and thus appears to be an isolated figure, not part of any community. She only attacks the Danes after they kill her son, and thus she displays a degree of maternal instinct. She is even more powerful than her son, providing Beowulf with an even greater opportunity to display his prowess. She possesses supernatural strength and determination, as displayed in her epic battle with Beowulf. She lacks any strong character beyond the simple identity of "evil monster."
Like her son, Grendel's mother is a descendent of Cain on the warpath for revenge. However, while Grendel primarily resented the Danes for their warm camaraderie, and the fact that, as humans, they have God's blessing, Grendel's mother hates the Danes because they hurt her son. Indeed, it is not until Grendel returns mortally wounded from his battle with Beowulf that Grendel's mother leaves the swamp and seeks her revenge.
Unlike her son, however, Grendel's mother receives comparatively little attention in the poem: she wreaks havoc in Herot, and then is swiftly dispatched by Beowulf. Even so, it's worth noting that, monstrous as she may be, she actually acts in very human-like way. Like a human mother, she is motivated primarily by the need to protect her offspring. As such, while Grendel's mother might be characterized as a villain in the poem, its impossible not to feel some semblance of pity for her, just as one would feel for any human mother who had recently lost her only son and companion.
Grendel's mother is as fierce as her son and seems even more vindictive. She is determined to get revenge for her son's death, thus exhibiting a strong maternal instinct. She too, is a descendent of Cain, the first murderer mentioned in the Bible. Thus, she is immediately connected with evil. She, like her son, are no match for the Danes who must call upon Beowulf for help against her attacks.