Peter Pettigrew
Background
Petter Pettigrew, "Wormtail", is a former follower of Voldemort. He betrayed the hiding place of his friends Lily Potter and James Potter and set Sirius Black up for his murder. Peter has secretly been disguised as Ron Weasley's rat Scabbers.
During the events of Harry Potter and the Prisoner of Azkaban (2004) played by Timothy Spall
Harry Potter goes to try and find Peter and Old Man in Portrait tells him to put out his wand light. Peter somehow walks past Harry without Harry seeing him. Later, Scabbers tries to run away and is transformed back into Peter and is caught. As Sirius and Professor Lupin are about to kill Peter, Harry stops them and says Peter needs to be taken to the castle and given to the authorities. They all exit the cavern under the Whomping Willow. Hermione Granger yells out when the full moon starts to peak over the horizon. Lupin starts to change into a werewolf and Peter manages to escape.
During the events of Harry Potter and the Goblet of Fire (2005) played by Timothy Spall
Frank Bryce walks to the Riddle house and hears Voldemort speaking to Peter. Frank looks into a room and sees Peter speaking to what appears to be a tiny figure in a chair. Voldemort tells Barty Crouch Junior to gather their old allies. Nagini slithers into the room and tells Voldemort that Frank is listening outside, and Voldemort uses a spell to kill Frank. Later, Harry and Cedric Diggory grab the Goblet of Fire at the same time and appear in the Riddle cemetery. Peter walks over with Voldemort and Voldemort has Peter kill Cedric. Peter dumps the tiny form of Voldemort into a cauldron, places a bone from the grave of Voldemort's father, cuts off his own right hand and then places blood from Harry into the cauldron and Voldemort is reborn. He sends out the Dark Mark and a group of Death Eaters appear among them the fathers of Crabbe and Goyle. Lucius Malfoy is also present and Voldemort gives Peter a metal hand to replace the one Peter cut off. Voldemort challenges Harry to a duel and as they are dueling, their wands appear to cancel each other out. The dead spirits of Frank Bryce, Cedric, James Potter and Lily Potter appear and James tells Harry to run to the Goblet, which acts as a Portkey.
During the events of Harry Potter and the Order of the Phoenix (2007) played by Charles Hughes
Professor Snape continues to train Harry in Occlumency, and Harry manages to get into Snape's memories and sees a memory of Snape's where he is picked on by James, Sirius, Peter and Lupin.
During the events of Harry Potter and the Half-Blood Prince (2009) played by Timothy Spall
Bellatrix Lestrange, Narcissa Malfoy, Peter, and Professor Snape have a meeting and Snape offers to help Narcissa. Before the meeting starts, Snape slams the door shut on Peter. Snape then creates an Unbreakable vow with Narcissa to help Draco Malfoy when he fulfills Voldemort's wishes.