Doug Hemmill is a former high school classmate of Matthew Reed.
Following a prom during his senior year of high school, Matthew and his girlfriend Claire Klein had a fight following Claire's suspicions that Matthew was flirting with another girl. Claire came crying to Doug and explained her situation. Doug was a longtime friend of hers but eventually became sexually attracted to her, and wanted to help her feel "relaxed", giving her hydrocodone. She accidentally overdosed it and died in Matthew's car, which she took in her frustration. Doug witnessed Matthew leave with Claire before she died and made it known to his friends that Matthew killed Claire.
Doug sees Matthew again at a high school reunion 20 years later. At a bowling alley, the screens show pictures of Claire's corpse as a cruel reminder to Doug of her death. Later at a dance, a mannequin of Claire falls along with the balloons, scaring the bystanders briefly. The gift bags contain hydrocodone prescriptions. Doug gets somehow gets a text from Claire, telling him to meet her at the chemistry lab.
There, Doug sees a suicide note signed in his name. Matthew comes in and confronts Doug with a gun, and Doug confesses that he gave Claire the hydrocodone. He begs for his life, saying that Matthew should take him to court since he's a prosecutor and what he's doing is injustice. Matthew claims that he lived 20 years in guilt but now that he knows the truth, killing Doug and letting him "rot" for another 20 years would only seem fair. He is about to shoot Doug when Reese and Shaw point their guns at him and force him to surrender.
Suddenly, Vigilance members open fire in the room. In the gunfight, Doug is shot in the leg, but survives along with Reese and Shaw, with the latter throwing an explosive chemical to take down the last remaining shooters. Doug is arrested while Matthew leaves to make a statement in court.