Peter Pettigrew, nicknamed Wormtail, is the only Death Eater known to have been in a House other than Slytherin (Gryffindor) while at Hogwarts. There, he was a close friend of Sirius Black, James Potter, and Remus Lupin, although he was the least intelligent and least talented of the group. With Sirius and James' help, Pettigrew becomes an Animagus, with the ability to transform at will into a rat. After leaving Hogwarts, Pettigrew joins forces with Voldemort, and in exchange for his own life becomes Voldemort's spy within the Order of the Phoenix, of which Pettigrew is a member. When the Potters know that their son, Harry, is Voldemort's target, Sirius suggests to them to use Pettigrew as Secret-Keeper because he does not believe Voldemort would ever suspect a "weak, talentless thing" like Pettigrew. Pettigrew betrays the secret to Voldemort, an act that leads to James and Lily's death (and ironically, Voldemort's near-destruction). Sirius seeks revenge on Pettigrew, but during the confrontation, Pettigrew publicly accuses Sirius of the Potters' deaths, murders twelve Muggles, and cuts off his own index finger before transforming into a rat, thereby framing Sirius for the betrayal of the Potters, as well as for his own murder and that of the bystanders. Pettigrew is "posthumously" awarded the Order of Merlin, and hides during the next twelve years. Wanting to keep an eye on the wizarding world, he masquerades as a rat, first as Percy Weasley's pet, and then as Ron Weasley's. In this form (named "Scabbers" by the family), he is missing a toe from one paw due to the finger he cut off.
Although Pettigrew appears in the first two books in the form of Scabbers, his identity is not revealed until Prisoner of Azkaban. When a photograph of the Weasley family appears in the Daily Prophet newspaper, Sirius recognises Pettigrew's Animagus form and escapes from Azkaban to track him down. The two confront each other in the Shrieking Shack, where Lupin and Black compel "Scabbers" to resume his human form. Pettigrew confesses his treachery, claiming to have committed it only to save his own life. With Sirius and Lupin about to take their revenge, Harry begs Sirius to turn Pettigrew over to the Ministry of Magic instead, to prove Sirius' innocence. Pettigrew escapes while being led out of the Shack when Lupin transforms into a werewolf. Harry's actions result in Pettigrew's owing him a life debt.
Pettigrew returns to the service of Voldemort, seeking him out in the forests of Albania and helping him to return to a feeble baby's body. He abducts a Ministry of Magic employee named Bertha Jorkins, who is able to provide Voldemort with valuable information. Pettigrew (almost always referred to as "Wormtail" hereafter) assists Barty Crouch Jr in overpowering Mad-Eye Moody, setting up the events in the fourth book, Harry Potter and the Goblet of Fire. In the climactic confrontation in that book, Wormtail murders Cedric Diggory on Voldemort's orders, and brews the complex potion to regenerate Voldemort, severing his hand as one of the ingredients. Upon his return to corporeal form, Voldemort replaces Wormtail's missing hand with a silver one that possesses five intact fingers and great strength. Despite his actions, Wormtail's fortunes remain low; in Harry Potter and the Half-Blood Prince Snape treats him as a servant, and in Harry Potter and the Deathly Hallows he is tasked with keeping watch over prisoners in the cellar of Malfoy Manor. While Harry and Ron are being kept there, Wormtail checks on the prisoners and is attacked. Wormtail begins strangling Harry with the silver hand, but when reminded by Harry that he once saved his life, Wormtail hesitates for a moment. The silver hand turns against him and strangles him to death as punishment for his moment of pity.
Pettigrew is portrayed by Timothy Spall as an adult, and by Charles Hughes as a teenager in the films. In Harry Potter and the Deathly Hallows – Part 1, Pettigrew is not strangled to death by his own silver hand in Malfoy Manor as in the book; he is instead struck by Dobby and collapses and is presumably killed. He only appears in a flashback in Harry Potter and the Deathly Hallows – Part 2.
Rowling has said in an interview that his peers let Pettigrew be their friend in a "slightly patronising way that he turned out to be a much better wizard and much better at hiding secrets than they ever thought he would be."
IGN's Joe Utichi also listed him as his 10th top Harry Potter character.