Peter Clarner was the boyfriend of Debbie Foley but was always deeply insecure, taking out his insecurities on Debbie in the form of emotional and physical abuse - which he then manipulated her into accepting, stating that she was to blame. In an effort to make himself more desirable to Debbie, Peter created a strange formula to "improve" himself. However, this formula only served to worsen his already paranoid and aggressive nature - transforming him into a superhuman brute similar to the iconic monster Mr Hyde.
Peter, under the influence of this formula, proceeded to brutally beat anyone he saw as a threat to his relationship with Debbie to death (or near-death) - his first victim was Jeff Walken, who Debbie had formed a friendship with and his next victim was arguably the most innocent of all - Mr. Platt, the school's guidance councilor, who was brutally mauled to death for simply doing his job.
Debbie tried to save Peter from himself by getting rid of most of the formula but only suffered his wrath as a result. When she started to befriend Oz, Peter once again went berserk and planned on killing Oz: tearing open the cage in the library Oz was being kept in, unaware that Oz was currently a werewolf, the two monsters fought briefly before Buffy tried to intervene and shoot Peter with a tranquilizer, once again Debbie came to her lover's aid and knocked Buffy aside - allowing Peter to escape.
Debbie would meet with Peter a final time in the school garden shed. She tried to convince Peter to leave Sunnydale so that Buffy wouldn't hunt him down for what he did - however Peter repaid Debbie's loyalty by brutally killing her for exposing him.
Moments later Buffy came to the shed and found Debbie's body, Peter ambushed the Slayer and prepared to kill her as well - only to be attacked by Angel, who had broken free of his bonds and - driven mad by years in a Hell dimension - was almost as feral as Peter himself, the two engaged in a battle but Angel, despite his viciousness, was still no match for Peter.
Peter lunged at Buffy again but was killed when Angel used his own chains to snap Peter's neck - as Peter died Angel apparently regained his lost sanity - following his death the student body of Sunnydale learnt of his and Debbie's relationship (minus the supernatural elements) and despite his misdeeds his death was greatly mourned by his former best-friend and short-time lover of Buffy, Scott Hope.