"But you know how much I like kids, Dumbledore."
-- Fenrir Greyback (HBP27)
Werewolf, low-level Death Eater, and friend of the Malfoy family. He killed for fun and especially enjoyed infecting children. Greyback was a follower of Voldemort since at least 1970s, though he was in it for the access to victims, not for ideological reasons. It was Greyback who contaminated Remus Lupin as a child (HBP16).
Described as a "big, rangy man with matted gray hair and whiskers." His black Death Eater's robes are "uncomfortably tight" and his dirty hands have "long yellowish nails." His voice is raspy and bark-like. Even when human Greyback has pointed teeth (HBP27). Greyback does not have a Dark Mark on his arm (DH23).
Possible name origin
Fenrir='Fenriswolf,' 'Fenrisulv,' 'Fenrisulf' the gigantic wolf of the God Loki in Scandinavian mythology. He was the brother of Hel (Queen of the Underworld) and when he gaped one jaw touched earth and the other heaven (Brewer's Dict.).
According to Lupin, "Fenrir Greyback is, perhaps, the most savage werewolf alive today. He regards it as his mission in life to bite and to contaminate as many people as possible; he wants to create enough werewolves to overcome the wizards." Greyback believes that "werewolves deserve blood" and preaches to fellow werewolves that they should revenge themselves upon the wizarding world (HBP16).
Greyback has been a Death Eater since at least 1970s, though he is in it for the access to victims, not for ideological reasons. According to Lupin, "at the full moon, [Greyback] positions himself close to victims, ensuring that he is near enough to strike. He plans it all." His primary value to Voldemort is as a threat to hold over parents -- Voldemort obtains cooperation from parents worried that he will "unleash" Greyback on their children. Indeed, it was Greyback who contaminated Lupin as a child when Lupin's father offended him (HBP16). Greyback is a lower-level Death Deater -- he does not have the Dark Mark on his arm (DH23).
In April 1997, Greyback killed the 5-year-old brother of the Montgomery sisters when he "got carried away." (HBP22)
The night that Dumbledore was killed, Greyback attacked Harry, but Harry (or someone unseen) petrifies him in time (HBP27). Bill Weasley wasn't so lucky. Greyback nearly killed him and mauled his face so badly that Madam Pomfrey could not restore it. Werewolf wounds are "cursed wounds" and never heal completely.
Greyback was also one of the snatchers who caught Harry, Ron and Hermione when they were on their Horcrux quest and took them to the Malfoy mansion. Greyback was wounded but not killed during the Battle of Hogwarts by a crystal ball thrown by Sibyll Trelawney. Later, Ron and Neville were able to "bring him down" (DH36).