Mercer was a colleague of Norman Jayden. He was a lawyer associated with the ARI program and worked with Jack Reilly before his apparent suicide. Prior to the events of Heavy Rain, he and Norman investigated Reilly's death together.
Background[]
As the Chronicles episode Mercer would have appeared in was never released, not much is known about him.
Mercer and Norman visit Jack Reilly's penthouse in Washington, D.C. when Reilly is found dead in his bedroom of an apparent drug overdose. Norman starts having flashbacks as they explore the penthouse, one of which Mercer triggers by turning on the TV in the lounge. Eventually, Mercer decides to wait for Norman in the car, and Norman has additional flashbacks that shed light on his history and relationship with Reilly as he continues looking around.
Once Norman has finished searching the penthouse and discovers why Reilly died, he goes to stand on the balcony to contemplate his findings. Mercer returns and Norman confronts him. The confrontation ends with Norman shooting Mercer, causing him to fall off of the balcony (presumably to his death).