As a teenager, Wormtail—Peter Pettigrew—was best friends with James, Sirius, and Lupin, but as a young adult, he began working for Voldemort and ultimately gave away James and Lily's location to Voldemort. He's interested mostly in serving whoever is the most powerful, which is what led him to Voldemort in the first place. He possesses one silver hand, which Voldemort gave Wormtail to thank him for sacrificing his original hand to bring Voldemort back to life. He owes Harry a debt, since Harry refused to kill him years ago, and Wormtail is forced to pay this back when Harry reminds him of it. Wormtail's silver hand strangles him, showing Harry that Wormtail did feel some sense of remorse and had the capacity to show mercy.
Wormtail Character Timeline in Harry Potter and the Deathly Hallows
The timeline below shows where the character Wormtail appears in Harry Potter and the Deathly Hallows. The colored dots and icons indicate which themes are associated with that appearance.
...though he no longer cares about the Horcruxes and thinks of how Dumbledore predicted that Wormtail would show Harry mercy. Ron and Dean join Harry to dig and, finally, Harry wraps... (full context)