Dr. Mercer was a Unitologist scientist who was sent by the Church of Unitology to study the Marker. Like the other members of the USG Ishimura and the Aegis VII colony, he goes insane shortly after coming in proximity to the Marker when it was brought on the ship. He was one of the first scientists to study the Necromorphs, and during his loss of sanity he began to believe them to be divinely created. He views their ability to re-animate dead flesh as a form of life after death, thus fulfilling a principle belief of Unitology.
Just prior the events of Dead Space, Dr. Mercer began experimenting on the crew of the Ishimura. Many of these experiments failed, and the subjects died. However by drilling a hole into the skull of one of his subjects and injecting biological matter directly into the brain, he was able to create the endlessly regenerating "super-Necromorph" known as the Hunter. He wanted to bring the Necromorphs back to Earth in order to infect the rest of Humanity, viewing the Necormorphs as the successors to the now obsolete Human race.
Having been thwarted on numerous occasions by engineer Isaac Clarke, Dr. Mercer eventually allows himself to be killed and infected, becoming a Necromorph himself, whom the player must kill. It is possible (and popular among fans) to kill the Infector before it transforms Mercer into a Necromorph, robbing him of the "second life" he forced upon his unwilling victims.