Wormtail

Peter Pettigrew (aka Wormtail and Scabbers) is an Animagus who betrayed Lily and James for Lord Voldemort, framed Sirius Black and became a Death Eater.

Harry's Destiny
Wormtail is first seen when he meets up with Lord Voldemort and shows him the location of the Potter family, in exchange for being a Death Eater.

Stone
Wormtail is seen as his rat form, Scabbers, and is revealed to be a pet of Ron Weasley. Scabbers seems to be an ally, as he helps Harry and Ron get to the Troll by going through the pipes.

Chamber
Wormtail is still seen as Scabbers, and can be summoned by Ron.

Prisoner
Wormtail plays an important role in Harry's third year. After hearing about Sirius Black's escape from Azkaban, Scabbers tries to escape many times, but Sirius drags Ron to the Shrieking Shack, in an attempt to kill Scabbers. After Harry and Hermione confront Sirius, Sirius explains that he is innocent and Wormtail was the one who framed him and betrayed Harry's parents. Scabbers escapes Ron's grasp again and runs around the room. However, Sirius and Remus eventually lure him out with a mousetrap and cheese and Wormtail is reverted back to his human form. They take him prisoner and attempt to take him to Azkaban, but when Lupin starts to turn into a werewolf, Wormtail turns into a rat and escapes.

He is seen later, when Dobby gives him a chest in it, in exchange for a few Galleons. Wormtail asks what's in it, and Dobby reveals that the Dark Lord is in the chest. Wormtail smirks and takes the chest.

Goblet
Wormtail has become a Death Eater again, and is seen with the infant Voldemort. When Harry and Cedric are teleported to the graveyard, Wormtail captures them and performs a ritual to resurrect Lord Voldemort. First, he puts the skull of Tom Riddle in a cauldron. Next, he sacrifices his own hand, which he takes off and puts that in the cauldron as well. Lastly, Wormtail puts Harry's glasses in the cauldron and puts the baby form of the Dark Lord in as well. Lord Voldemort is resurrected and summons the Death Eaters, while Wormtail hides behind the carriage which Wormtail used to transport Voldemort's infant body.