While most of his origin story is unknown, Doomguy was a Space Marine who was sent to Phobos, one of the twin moons of Mars, as punishment for disobeying his commanding officer, as well as for beating him to death. On Phobos, Doomguy discovered a magical gateway that leads straight to Hell. Ever since then, Doomguy has been responsible for stopping the demonic armies of Hell from invading Earth.
Doomguy is the only character on Death Battle without an official name, since he was never given one.
Doomguy first came to being within the 1990s era, in his case being December 10, 1993 due to the fact that DOOM first came out for MS-DOS & Mac OS.
Doomguy isn't ever referred to by name, due to the fact that the original game's co-designer John Romero said this was to increase player immersion. Romero said, "There was never a name for the DOOM marine because it's supposed to be YOU."
Doomguy also appears in the comic book of DOOM was issued in May 1996, produced by Tom Grindberg of Marvel Comics as a giveaway for a video game convention.
Doomguy's name in the film version of DOOM was actually named John "Reaper" Grimm, it should be pointed out that the film is non-canon.
Doomguy also has a sister in the film version of DOOM named Dr. Samantha Grimm.
Doomguy is left-handed in the first two DOOM games.
In the novel versions of DOOM & DOOM II, Doomguy's name is Flynn Taggart (the novels where shunned by fans).