Pierce Moncrieff is a hunter who was affiliated with the Men of Letters. However, he betrayed them and his fellow hunters by giving the Alpha Vampire, their location in America. After the Alpha was killed, Pierce was taken into custody for his betrayal but his fate is unknown.
Pierce is a hunter from Baton Rouge, Louisiana, who was recruited by the British Men of Letters. He is first seen at the British Men of Letters compound inside the conference room with all the other operatives. Mick Davies introduces Pierce to hunters Mary and Sam Winchester before introducing the three of them to British Men of Letters staff members Alton Morehead and Serena Colman.
Pierce listens to the group discuss Project V, an project centered around eradicating all of the vampires in America. After the discussion, Pierce tells Sam he heard about him from Rufus Turner, saying the two of them worked on a case together down the Atchafalaya, a case Sam is familiar with. Pierce goes on to say how he misses Rufus.
Sam is later shown talking about Pierce with Mick, noting how he isn't the best member of the group. Mick laments that aside from Sam and Mary, the British Men of Letters have had little luck attracting the "top shelf of American hunters".
The compound is soon raided by a nest of vampires. The group manage to lock the place down and capture one of them, who laughingly reveals that his father told them where they were. It is then that the operatives realize their intel has been wrong, since they weren't expecting the Alpha to be in the area.
When a decision is made to use the Colt, Mary, Sam and Pierce are sent back outside the conference room to keep the vampires from entering while Mick and Alton create the bullets. When Sam is separated from them, Pierce knocks Mary out and goes back to the conference room and bangs on the door, demanding entrance.
Alton complies, and asks him what happened. Pierce simply tells the three that it's bad out there before the Alpha Vampire himself appears and kills Alton and then Serena. Sam and Mary learn through a radio transmission that Pierce is actually working for the Alpha, and is acting as a mole in the group by betraying the British Men of Letters. The two manage to return to the room while the Alpha confronts Mick over what he has done to the Alpha's children.
As Pierce watches on, Mary comes up behind Pierce and knocks him out in return. Sam and Mick work together and successfully kill the Alpha using The Colt. In response to his betrayal, Arthur Ketch bounds Pierce in chains and takes him custody, promising Sam and Mary that Pierce will be punished accordingly. Both Sam and Mary respond by saying "Good".
Presumably, Pierce was tortured extensively by Ketch.