John Grimm

John "Reaper" Grimm is the protagonist of the 2005 film Doom.

History
John "Reaper" Grimm is the son of UAC scientists who were killed in an accident during the early excavation of a Martian dig site. Reaper abandoned his scientific heritage and joined the military to forget about this personal tragedy, eventually becoming a member of the elite Rapid Response Tactical Squad (RRTS).

Grimm, his commanding officer Asher Mahonin, and the other members of the RRTS were dispatched to the UAC Olduvai Research Facility to investigate the disappearance of several scientists. Grimm also reunited with his estranged sister Dr. Samantha Grimm, who is a researcher at the facility. Ultimately, he and his squad were pitted in a confrontation against genetically-engineered monsters created by an ancient Martian retrovirus accidentally released by the UAC.

Grimm clashed with Mahonin over his commanding officer's increasing aggression that is made worse by the latter's orders to his men to kill any survivors in the facility to prevent the virus from leaving Mars, which Grimm refused. This is made worse when Mahonin killed Mark Dantalian for insubordination.

The RRTS were then completely annihilated by the mutants and leaving Grimm the sole survivor of his squad. He was fatally wounded by a ricocheted bullet and was saved by his sister, who injected him with the C24 serum; granting him superhuman abilities while retaining his mental stability. Grimm goes on the offensive and single-handedly kills every single monster in the facility. He was eventually attacked by a mutated Mahonin, who was also accidentally infected with the experimental chromosome in an earlier monster attack. After an intense battle, Grimm defeat Mahonin by throwing him through the Arc portal, along with a grenade after him which kills Mahonin and destroying the Mars facility. Grimm leaves the base with his sister.

Personality
Grimm's personality can be seen as rather angsty, as he is still dealing with his guilt over his parents' deaths and his concern for his sister, Dr. Samantha Grimm, a researcher at the UAC Mars Facility. However, like his computer-game renditions, he has no problem killing multiple demons and even his own commanding officer, Sarge, after the latter begins to murder unarmed civilians in order to contain the outbreak.

Notably, at the end of the film, Grimm is injected with the Martian genetic material. Instead of turning into a monster, he instead gains superhuman strength, reflexes, and regenerative abilities. These powers allow him to stride through the infested base singlehandedly mowing down a small horde of demons including many zombies, several imps, a hell knight, and a demon (all seen by the audience in the game's classic first person perspective). This plot device is apparently the film's way of incorporating the berserk powerups in the games (see Berserk pack and Berserker).

Trivia

 * Grimm is the film's equivalent of the Doomguy. The only differences that make him stand out from the protagonists in the games is due to having an actual name and interacting with other characters via his own dialogue. His opposition with Sarge for ordering him to kill innocent civilians mirrors the protagonist's backstory in the original Doom.