"My mind ... my personality ... were literally ... burned out of me." - Peter to Derek Hale
"I heard there was a party. Don't worry. I invited myself." - Peter to Derek and Lydia Martin
Peter Hale is a born werewolf from the Hale Family whom lived in Beacon Hills, California, the younger brother of the late Talia Hale and Derek Hale's uncle. During the Hale House Fire, six years before the series, Peter had survived but was rendered comatose at Beacon Memorial. In Season 1, Peter is revealed to be the alpha werewolf who had killed Laura Hale, his niece, to gain alpha rank, lured Derek back to Beacon Hills and had bitten Scott McCall. He kills all the criminals involved in his family's massacre, bites Lydia Martin and finally, kills Kate Argent, the perpetrator of the arson. He is defeated by Stiles Stilinski, Allison Argent, and Jackson Whittemore using Lydia's Molotov cocktails and is then killed by Derek. In Season 2, Peter haunts Lydia as his bite awakened her latent banshee powers. He possesses her in order to orchestrate his resurrection, coming back to life as an omega. Peter provides the necessary information to turn Jackson from a kanima into a werewolf in exchange for being in Derek's Pack. In Season 3, he provides information against threats to the pack and helps to eliminate them, most notably making sure Jennifer Blake was really dead so she would not come back to wipe out the pack. He also killed The Mute who attempted to kill him and threatened to kill Derek. In the process of helping to separate the Nogitsune from Stiles, he made a deal with Lydia to find out what memories had been stolen from him by Talia and learns he'd fathered a daughter, Malia Tate, with the werecoyote assassin the Desert Wolf. In Season 4 Peter makes a deal with Kate, now a resurrected werejaguar, intending to double-cross her in order to test Scott to see if he's willing to kill in order to defend his pack. When this fails, Peter plays the villain the others expect him to be in order to see if Scott would be willing to kill him instead. However, Scott fails Peter's test by defeating him without killing and incarcerates him in Eichen House instead. In Season 5 it is revealed that Peter is no longer in Eichen House, although it is unknown what happened to him and why.
"No one would ban me from anywhere. I'm too good-looking." - Peter to Derek Hale
Peter places an extremely high importance on protecting his pack from threats and doesn't care if he has to take the less moral road to achieve that. He judges an alpha's worth by their willingness to kill threats to the pack before the threat is able to act on them and has found both Scott and his sister Talia lacking in this respect.
"You. ... All for power."
"For my family's power. To be rightfully inherited by me. Not some teenage idiot, so incorruptible, he wouldn't shed the blood of his enemies even when justified. You don't deserve your power. Not power like this."
Peter is well-versed and has vast knowledge and experience regarding the supernatural world. While he was an alpha, he could transform into a monstrously strong wolf beast, with enhanced speed and strength. He is a master at mind-melding and was able to come back from the dead by possessing Lydia in order to perform a ritual.
Peter is a hereditary werewolf born into the Hale Family, a prominent pack of werewolves. His older sister Talia was the alpha for their family+. Peter became uncle to her children, Laura, Derek and Cora Hale++.
At some point, Peter fathered a child with an assassin, "The Desert Wolf". The woman gave birth, but Talia wiped Peter's memories of his fathering the child and those of Desert Wolf, and snatched the as-of-yet unnamed child+++.
In Derek's youth, Peter looked out for him when the Argent werewolf hunters arrived in town, the same time as a congregation of packs were arriving for a meeting with Talia. Peter was proven to be a terrible influence over Derek. He was a schemer and manipulator. He tried to talk Derek into the idea of turning Paige to remove any inhibitions between him and her and set Paige up to be bitten by Ennis without her consent. The bite failed, leaving her in such extreme pain that she asked Derek to mercy kill her to spare her any additional suffering +. Following Paige's death, Peter advised Derek to use anger, emotion, rage as an anchor to control his shifts, further shaping Derek into who he'd become by seasons 1 and 2 +.
After six years, on the night of a full moon, Peter came out of his coma. He set about orchestrating his revenge on all the people involved with setting the fire and decimating his family+. With the help of his nurse, Jennifer, Peter set about tracking down all the people who'd set the fire. His niece Laura is lured back to Beacon Hills and into the woods where Peter kills her, stealing her alpha status in order to heal completely and to carry out his revenge++.
Becoming an alpha and his experience as a werewolf allowed Peter to shapeshift into a hulking, overlarge wolf-beast. Peter then sought out someone new to bite and transform so he could rally a new pack to further implement his plan for revenge.