Contrast Beowulf and Unferth. What function does unferth serve in the poem?

Yes, he's a foil. He contrasts with Beowulf, and he has committed the same crime as the one that Grendel's ancestor committed that has made Grendel what he is--fratricide. At the feast just before the coming of Grendel's mother, when Wealhtheow says that she knows she can trust Hrothgar's nephew, Hrothulf, to protest the inheritance rights of her sons, it's ominous that Hrothulf is sitting beside Unferth. The hint that Hrothulf will dispossess and maybe even kill his young cousins makes Beowulf's refusal to accept the Geatish throne in place of Hygelac's son shine all the more brightly.

However, Unferth has another function in the poem. By belittling Beowulf's performance in the swimming race, he provides an opportunity for Beowulf to give the true account of the race--in effect, to state his credentials, his qualifications to fight Grendel. And when Beowulf prepares to go down into he swamp to fight Grendel's mother, Unferth lends him an heirloom sword--an act that can be interpreted as acknowledging that Beowulf is a better man. I took this excerpt from another site. I thought it was more detailed than mine. Please check out the source link below.