Ethan: "We know about the lunar eclipse, so don't think Kali's gonna sit around waiting for it to level the playing field. She's coming, and my brother is coming with her."
Peter: "Good enough for me. Derek?"
Derek: "You want me to run?"
Peter: "No, I want you to stay and get slaughtered by an Alpha with a psychotic foot fetish. Of course I want you to run! Sprint, gallop, leap your way the hell out of this town!"

The relationship between Omega Werewolves Derek Hale and Peter Hale.

Derek and Peter have had a back and forth relationship that has gone from being familial to enemies. In their youth, Peter considered Derek to be his favorite nephew and frequently "looked out" for him, though his methods for doing so were questionable, such as his frequent spying on Derek while he was with his high school girlfriend, Paige Krasikeva.

After the Hale House Fire caused by Kate Argent and her many co-conspirators, Derek and his older sister, Laura Hale, the only uninjured survivors, fled to New York to evade the Hunters while Peter was left in a coma in Beacon Hills Memorial Hospital. The two reunited in Season 1 of the series when Derek brought the newly-turned Werewolf Scott McCall to visit Peter so that he would know just how dangerous the Argent Family could be. However, it wasn't until the end of the season that Derek learned that not only had Peter slowly come out of his coma, but that he had also killed his sister Laura in order to steal her Alpha power so he could fully heal himself. While Derek was willing to join Peter's pack until this point, upon learning of his role in Laura's death, Derek killed Peter and stole his power, becoming an Alpha himself.

In Season 2, Peter, who had used his connection to Lydia Martin (who had been bitten by Peter, triggering her latent Banshee nature) to force her to perform a ritual to resurrect him with a blood sacrifice from Derek and the light of the Worm Moon. After he was reborn, Peter convinced Derek to allow him into his pack, as he needed an Alpha and Derek needed a Beta after his former Betas Erica Reyes and Vernon Boyd took off in search of a new pack. After Peter helped Derek figure out how to turn Jackson Whittemore, who was turned into a Kanima after receiving the Bite from Derek, back into the Werewolf he was meant to be, Peter became a member of the Hale Pack.

Peter then became a mentor of sorts to Derek, Scott McCall, and the other members of the Beacon Hills packs, as he knew just as much, if not more, than the other allies and main sources of supernatural information, Alan Deaton and Chris Argent. However, after Peter killed Jennifer Blake, a Darach, on the Nemeton, it became clear that he was just biding his time until he could regain Alpha status and gain more power. He continued to assist the packs in various supernatural issues, such as figuring out how to defeat the Nogitsune and the Oni, why the resurrected Werejaguar Kate Argent de-aged Derek, and who the architect of the Deadpool known as The Benefactor was. Peter finally saw his opportunity to regain the power he believed he deserved when he teamed up with Kate to turn Scott into a Berserker in order to have someone in the McCall Pack mistakenly kill him in hopes that he would eventually gain Scott's Alpha powers.

Unfortunately for him, Derek assisted the pack and its allies in neutralizing Peter with yellow wolfsbane and imprisoning him in the supernatural unit of Eichen House. Derek and Peter were not able to reunite again until the Season 6 finale, The Wolves of War, when Derek, who had been traveling in Brazil and the southern United States, returned to his hometown to assist Scott, Peter, and the rest of the McCall Pack in fighting the Anuk-ite and the rabid Hunters who had been corrupted by the fear the Anuk-ite intensified. It was then shown that Derek had regained his status as a McCall Pack member and Peter was a solid ally of the pack as well.

Throughout Teen Wolf

According to Peter, he was once very close to Derek and even went as far as to say that he was Derek's best friend. However, considering Peter was retelling the past from his point of view and was implied to be an unreliable narrator, it's unknown how true the statement was. It is at least revealed that at best, Peter had mentored Derek and at worst, Derek found him annoying and even once threatened to have him banned from school grounds, though it's unknown if this was always the state of their relationship or if Derek was being facetious.

Derek fell in love with a girl called Paige Krasikeva, who was a fellow student at Beacon Hills High School and of whom Peter approved tremendously; while he found the rest of Derek's friends to be annoying teenagers, he believed Paige was perfect for him. Playing on Derek's insecurities, Peter mentioned that Paige would likely break up with him when she found out he was a Werewolf and planted the idea that one of the visiting Alphas seeking counsel from Talia Hale, Derek's mother and Peter's sister, could give Paige "the Bite" and make her one of them so they could always be together.

Out of all of the Alphas attending the summit, Peter had chosen Ennis, who had recently lost one of his Betas to Hunters, to be the one to turn Paige; Ennis agreed to this plan, not only to fill the void with a new Beta but to also "get in good" with Talia, who was a pillar of the Werewolf community. Though Derek initially agreed to this plan due to his fear of losing Paige, he lost his nerve when he heard Ennis attacking her at school and attempted to intervene, but it was no use-- Ennis easily overpowered him and pinned him to the floor to see that he had already bitten her.

Derek took Paige to the Nemeton's root cellar, where it was revealed that she was dying as a result of her body rejecting the Bite. Shortly afterward, Peter arrived to find Derek cradling the dying Paige in his lap and became remorseful for his role in what had happened. After Derek was unable to take her pain, Paige begged him to end her suffering, and he mercy-killed her by snapping her spine, causing his gold Beta eyes to turn blue. Peter then took Paige's body and left her in the woods to be found and ruled another one of Beacon Hills' many animal attack victims.

Sometime after this incident, Derek forced himself to attend his basketball game, but had no choice but to hide in the boys locker room when he struggled to control himself during the full moon. Peter attempted to teach him the traditional way of the Hale Family by using the Triskelion Medallion and the "Alpha, Beta, Omega" mantra, but it was unsuccessful. From then on, Peter taught Derek to use anger as an anchor and source for power, given him control and power at the same time in the same manner that he himself used to keep control.