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John "Reaper" Grimm is the son of UAC scientists who were killed in an accident during the early excavation of the 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 are dispatched to the UAC Mars Facility to investigate the disappearance of several scientists, which ultimately pits them in a confrontation against genetically engineered monsters created by an ancient Martian retrovirus released by the UAC.
When Mahonin ordered his men to sanitize any survivors in the facility to prevent the virus from leaving Mars, Grimm refused to follow his orders. After Mahonin was captured by a mutant, Grimm was the sole of the RRTS. He was wounded by a ricocheted bullet in which his sister injects him with the C24 serum; granting him superhuman abilities while retaining his mental stability. Grimm goes on the offensive and kills every single mutant on the base and encounters a mutated Mahonin afterwards. After an intense battle, Grimm defeat Mahonin by throwing him through the Arc portal, along with a grenade which kills Mahonin.
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).
- Grimm also differs from the protagonists in the games due to having an actual name and interacting with other characters via his own dialogue.