The Doomguy is the protagonist of the Doom series of video games created by id Software, and its sequels and spin-off media.
In all the games, he is a former marine, wearing green combat armor, working for the Union Aerospace Corporation. He never speaks (there is usually no one around to talk to). In the games, he is never referred to by name. The game's designer, John Romero, said this was to increase player immersion: "There was never a name for the DOOM marine because it's supposed to be YOU."
In the Doom novels, the main character is referred to as Flynn "Fly" Taggart. However, Tom Hall's original design draft, also known as The Doom Bible, suggested his name was Buddy Dacote ("Dacote" standing for "Dies at conclusion of this episode", which is indeed the fate of the character, although he continues his adventure in the second episode). In the Wolfenstein RPG, it is implied that Doomguy is the descendant of William "B.J." Blazkowicz. In the game Blazkowicz defeats the "Harbinger of Doom" (after taking off one arm and one leg), the creature states that he will return in the future to confront his descendants (this is a reference to the Doom guy's confrontation with the Cyber Demon).
On the box art for the original Doom, the Doom Marine is portrayed as a muscular man wearing green armor as well as a light gray space helmet that conceals his facial features. He is firing a plasma rifle and fighting off a demon that is grabbing his left wrist. The introduction screen of Doom is slightly different, as he still has his plasma rifle but is also clutching a shotgun in his left. The player's in-game avatar, as seen in multiplayer and in the ending to Doom II, is based on this depiction. Colors in multiplayer include green, red, brown and indigo . In Doom 64, the Marine is portrayed as appearing to have lost some weight, with slightly modified black armor and a black helmet with an antenna. His visor is blue rather than grey.
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Official Site: id Software
Links: Wikipedia: Doomguy Answers.com: Doomguy Doom Wiki: Doomguy