Matthew P. Reed is a prosecutor, former student of East Springs High School, and POI.
Following a prom during his senior year, Matthew had a fight with his girlfriend Claire Klein when she suspected him of flirting with another girl; Claire ran to classmate and friend Doug Hemmill and explained her situation. Doug was sexually attracted to Claire and gave her hydrocodone to help her feel more "relaxed", and she left the prom, taking Matthew's car in her frustration. Inside, she accidentally overdosed with hydrocodone and died, but based on the circumstances, Doug made it known to classmates that Matthew was the one who killed Claire since she was in his car when she died.
For the next 20 years, Matthew also decided to accept the fact that it was his fault Claire died, living out of guilt. When he learned of a high school reunion taking place, he started planning revenge on Doug.
The Machine provides Matthew's number as he becomes a perpetrator, and Finch sends Reese and Shaw to investigate, with them posing as former students Frank Mercer and Betty Harris. Matthew says hi to them and notices Betty's change in appearance.
During a bowling alley party, the screens suddenly show Claire's corpse as a reminder of her death; Matthew appears disturbed and leaves. Later at a ballroom dance, Matthew deduces that Shaw isn't Betty, and a mannequin of Claire drops with the balloons to frighten the audience, and the present bags are filled with hydrocodone. Doug somehow gets a text from Claire to meet her at the chemistry lab.
Doug sees a suicide note written in his name there, and Matthew appears with a gun, revealing that he was behind the contraptions and the note to make it look like Doug confessed to killing Claire and then committed suicide. Doug explains what he did, but begs Matthew to spare his life, saying he should be taking him to court as he is a prosecutor. Matthew says that he lived 20 years in guilt, but now that he knows the truth, Doug deserves to "rot" for another 20 to make it fair. He is about to shoot Doug when Reese and Shaw appear with guns, forcing him to surrender.
Vigilance soldiers burst in and open fire; Shaw and Reese have a long gunfight with them and Shaw finds some chemicals that Matthew mixes, and Shaw throws the mixture at the last remaining men, and it explodes.
Doug is arrested and Matthew prepares to go to make a court statement. He says goodbye to Shaw.