Fenrir Greyback was a notoriously savage and cruel werewolf who allied himself with Lord Voldemort during the Second Wizarding War. Unlike most werewolves who regretted their condition, Greyback embraced his lycanthropy to a terrifying degree, developing a preference for attacking children and a taste for human flesh even in his human form. His primary goal was to create a werewolf army to overthrow the wizarding community. Greyback is infamous for being the werewolf who attacked and infected a young Remus Lupin, and he played a significant role as a fearsome combatant and leader of the Snatchers for the Death Eater regime.
Little is known of Greyback's early life, but he was already considered the most savage werewolf alive by the 1960s. He specialized in positioning himself near his victims to ensure he could attack them. His life became inextricably linked with the Lupin family when Lyall Lupin, an authority on Non-Human Spirituous Apparitions at the Ministry of Magic, identified Greyback as a werewolf during an inquiry. After Lyall described werewolves as “soulless, evil, deserving nothing but death,” Greyback sought revenge. He forced his way into five-year-old Remus Lupin's bedroom and viciously attacked him, ensuring Remus would become a werewolf himself. (Pottermore)
Greyback's activity escalated significantly during the Second Wizarding War. He was seen in Knockturn Alley at Borgin and Burkes, where he accompanied a group of Death Eaters to intimidate Mr. Borgin on behalf of Draco Malfoy. Greyback used the threat of his nature to ensure Borgin's cooperation. He later participated in the Battle of the Astronomy Tower at Hogwarts. After the Death Eaters entered the castle via the Vanishing Cabinet, Greyback launched a brutal attack on Bill Weasley, mauling his face severely. Because Greyback was not transformed into a wolf at the time, Bill did not become a full werewolf but was left with permanent scars and a new fondness for very rare steaks. During the same battle, Greyback was about to attack Harry Potter before being blasted away by Severus Snape. Following Lord Voldemort's rise to power, Greyback was given a prominent role as the leader of a pack of Snatchers. This group was tasked with hunting down Muggle-borns, blood traitors, and any others who defied the new regime, in exchange for gold. It was Greyback's pack that captured Harry Potter, Ron Weasley, and Hermione Granger in the Forest of Dean after they broke the Taboo on Voldemort's name. He took the trio to Malfoy Manor, where he expressed a disturbingly eager desire for Bellatrix Lestrange to give him Hermione Granger. Greyback was a key combatant in the Battle of Hogwarts. During the fighting, he was seen attacking Lavender Brown in the castle's entrance hall. He was repelled when Professor Trelawney dropped a crystal ball on his head, stunning him. He later recovered and was seen fighting Ron Weasley and Neville Longbottom, who defeated him. His ultimate fate in the battle is not specified in the novels, but he was left incapacitated.
Greyback was described as a large, thin man with matted grey hair, a rasping, barking voice, and an unwholesome smell of dirt, sweat, and blood. Even in human form, he possessed wolfish characteristics, including long, yellowed fingernails and pointed teeth. He typically wore filthy, torn Wizarding robes. His personality was defined by his extreme sadism and brutality. Unlike most werewolves, Greyback took immense pride in his condition. He made it his mission to bite and contaminate as many people as possible, with a particular focus on children, whom he planned to raise to hate wizards. He enjoyed the taste of human flesh even when not transformed, a trait that set him apart from other werewolves and horrified even his Death Eater allies. Though he served Lord Voldemort, he was not a true Death Eater and did not possess the Dark Mark, a reflection of the prejudice against his kind even within Voldemort's ranks.
No specific possessions are mentioned. Greyback is never noted to carry or use a wand, relying instead on his physical and lupine abilities for combat and intimidation.