Fenrir Greyback is a werewolf. A big, rangy man with matted grey hair and whiskers, his voice is like a rasping bark. His hands have long, yellowish nails and he has pointed teeth.
Fenrir is one of the Death Eaters who enter Hogwarts through the pair of Vanishing Cabinets. He joins the Carrows, Draco Malfoy and Professor Dumbledore on the Astronomy Tower. He was not expected to be part of the group at Hogwarts, but tells Dumbledore he couldn't resist the thought of so many humans in one place. Fenrir is unusual in having developed a taste for human flesh that is not limited to his monthly time as a werewolf. When Draco does not kill Dumbledore immediately, Fenrir offers to do the honours, and moves towards the Headmaster with his hands outstretched and teeth bared. He is blasted out of the way by the fourth Death Eater, hitting the ramparts and staggering, looking furious.
Greyback is present at Malfoy Manor. The werewolf is offered the chance to kill Hermione by Bellatrix, but is prevented from doing so by Ron, Harry and Dobby.