Peter Pettigrew is a friend of James Potter, Sirius Black and Remus Lupin since the school times. As Wormtail he was one of the creatores of the Marauder's Map and an unregistered animagus (he could change into a rat). In the third book it turns out that Ron's rat - Scabbers is in fact Peter.
Peter Pettigrew was a student at Hogwarts at the same time as James Potter and Sirius Black. A fat little boy, he hero-worshipped the pair, but was never in their league, talent-wise. Pettigrew was hopeless at duelling at school.
On the day he died, Muggle eye-witnesses said that Pettigrew cornered Black, sobbing and asking Black how he could have betrayed the Potters. Witnesses say he then went for his wand but Black was too quick and blew him to smithereens. Peter Pettigrew was posthumously awarded the Order of Merlin, First Class.
Lupin tells the trio that Pettigrew was one of his three closest friends at Hogwarts. Alongside James Potter and Sirius Black, Pettigrew worked to become an Animagus when he was a student, so that the friends could accompany Lupin during his transformations. Pettigrew's nickname was Wormtail.
Pettigrew is a very short man, with thin, colourless hair and a large bald patch. He has grubby looking skin, a pointy nose, tiny eyes and when he first transforms he has the appearance of a plump man who has lost weight very quickly. He is also missing the index finger on his right hand. After his forced transformation from Scabbers, Pettigrew seems nervous, and his eyes constantly dart between the occupants of the room and the door. When he sees Lupin and Black he greets them nervously. Pettigrew pleads with Lupin to protect him against Black, seeming scared of his supposed murderer.
Pettigrew hysterically tries to persuade those in the room that Black is trying to kill him. He attempts to defend himself against Black's accusations, initially denying that he was ever a spy for Voldemort or that he was named the Secret Keeper for Lily, James and Harry. He eventually admits that he had been passing information to Voldemort, but tries to justify his decision, telling Black that the Dark Lord would have killed him if he hadn't agreed to be a spy. He shows himself to have been a weak man, more concerned with his own safety than that of his friends.
Pettigrew makes his way around the people in the room, begging each of them to help him. He flings himself at Harry in gratitude when Harry refuses to let Black and Lupin kill him.
Pettigrew is shackled between Lupin and Ron when they leave the Shack, walking awkwardly in single file though the tunnel. When they leave the tunnel at the Whomping Willow, they start to make their way towards the castle, still chained together. Pettigrew is silent except for the occasional whimper as they start to walk towards Hogwarts. He, Ron and Lupin stop abruptly when they are suddenly bathed in moonlight. When Lupin transforms and wrenches free of the manacle binding him, Pettigrew is left shackled only to Ron. As Harry, Ron and Hermione watch Sirius and Lupin fight, Pettigrew makes a dive towards Lupin's wand, which has been abandoned on the ground. Pettigrew attacks Ron and Crookshanks with the wand, transforms into a rat, and scurries away across the grass.
Wormtail" is staying in the Riddle House with Voldemort. Timid around his leader, he acts as a servant, fetching food and waiting on him. Wormtail is curious about how long they will be staying in the Riddle House, and doesn't understand why they must wait to implement their plan until the Quidditch World Cup is over. He seems nervous of Voldemort's plan; before he loses his nerve, he questions whether it can be done without Harry Potter. He is quick to deny that he has any concern for Harry, but thinks that their plan could be enforced more quickly if they were able to use any other witch or wizard. Wormtail tells Voldemort that, if he were allowed to leave for a short while, he could return in as little as two days with a suitable person. He denies that he wants to leave Voldemort, but is not believed: Voldemort says that he can always tell when Wormtail is lying, and knows that he revolts the man.
Wormtail was the one who found Voldemort, and who brought Bertha Jorkins to him. Although finding Bertha was extremely useful to Voldemort, at the time Wormtail was unaware of how useful she would be. Voldemort tells Wormtail that he needs the assistance of someone with brains, and whose loyalty has never wavered, neither of which is true of him.
Wormtail ushers Frank Bryce in to the room in the Riddle House after Nagini informs Voldemort that the Muggle is outside the door. He is frightened about the idea of revealing Voldemort to Frank, but obeys and turns his chair to face Frank. He watches as Voldemort confronts the Muggle, ultimately killing him.
Wormtail carries Voldemort through the graveyard towards Harry and Cedric. He is ordered by Voldemort to kill Cedric Diggory, then drags Harry towards the grave of Tom Riddle Sr. He ties Harry from neck to ankles to the headstone, hitting the boy when he struggles. His fingers tremble as he checks Harry bonds, then leaves to fetch a large stone cauldron, which he drags to the foot of the grave. Wormtail lights a fire at the bottom of the cauldron, and begins to prepare a potion for the Dark Lord. He lowers Voldemort into the cauldron, then speaks to the night, 'Bone of the father, unknowingly given, you will renew your son!’. The ground by the grave cracks and a fine trickle of dust rises at Wormtail's command, falling into the cauldron. He whimpers as he draws a dagger from inside his robes, his voice breaking as he recites 'Flesh of the servant, willingly given, you will revive your master.' He grips the dagger tightly as he cuts off his own right hand, dropping it into the cauldron. Wormtail gasps and moans with agony after his actions, then stands before Harry to collect the 'Blood of the enemy' cutting the crook of Harry's right arm and collecting the blood in a glass phial. He staggers back to the cauldron and pours the blood inside, and drops to his knees once his job is done, cradling the bleeding stump of his arm. Wormtail helps to robe the newly risen Lord Voldemort after his master emerges from the cauldron.
Wormtail approaches Harry and raises his new silver hand, releasing Harry from the bonds tying him to the gravestone. He walks out of the circle to the place where Cedric's body lies and picks up Harry's wand, returning it to the boy. He then resumes his place in the circle of Death Eaters.
Barty Crouch Junior reveals that Wormtail helped him to prepare the Polyjuice Potion and kidnap the real Alastor Moody. After the successful switch, Wormtail moved into the Crouch home with Voldemort, enabling him to watch over the Imperiused Mr Crouch and to care for his master at the same time. He was not watchful enough of Mr Crouch, which allowed Mr Crouch to successfully escape from the house.
Wormtail is sent to Severus Snape's house on Spinner's End by Lord Voldemort, to assist his fellow Death Eater. He brings drinks for Bellatrix Lestrange, Narcissa Malfoy and Snape, then is sent out of the room.