Gunnery Sergeant Asher "Sarge" Mahonin is the leader of the Rapid Response Tactical Squad and the main antagonist / false protagonist of the film.
Sarge is informed by the General of the distress signal sent out by the Union Aerospace Corporation. Once the group arrives on Mars, Sarge and Duke search through the Weapons Research Facility. After finding Dr. Todd Carmack alive, he takes the severed arm of a deceased UAC employee, which he uses for DNA verification to gain access to the Bio Force Gun (He nicknames it "Big Fucking Gun" due to the abbreviation BFG)
After returning to Earth, Sarge becomes tense and demanding and begins ordering his men to kill every remaining survivor in the facility to insure that the C24 virus does not spread to the surface. When The Kid informs Sarge that he spared several survivors he found, Sarge violently murders The Kid, which causes Reaper to turn against him. Sarge is dragged away by an Imp, whilst yelling at Reaper, telling him how he wasn't meant to die. Sarge presumably managed to kill the mutants that dragged him away and escape, showing up later in the film.After Reaper is shot by a ricocheted bullet, Samantha injects the 24th chromosome into him, but instead of turning him into a mutant, he develops superhuman abilities while retaining his mental stability. Reaper goes on the offensive and kills every single mutant on the base and encounters Sarge afterwards. Sarge's physical appearance has not yet changed, but at this point his strength has developed far superior and sanity almost gone. Sarge tells Reaper he has killed the survivors The Kid found in the sector. He asks how much ammo Reaper has left and then fires his BFG at Reaper, who dodges the shot. After an intense battle, Reaper gains the upper hand and throws Sarge through the Arc portal, along with a grenade. Sarge is killed by the blast.