"Doomguy" is the name given by fans to the anonymous space marine that the player controls throughout every game in the Doom series. Next to nothing is known about him specifically, apart from the incident that got his ass to Mars: he not only refused to follow a direct order to fire on innocent civilians, he put the officer who gave that order in a full-body cast. He couldn't exactly be court martialed for this, as the order in question was totally illegal, so he was "promoted" to a position in the least interesting post in the entire military: guarding the scientists and eggheads studying the ancient ruins on Mars.
As it turned out, this quickly became the most exciting post in the solar system, as the scientists accidentally opened a portal to Hell while experimenting with the alien technology, releasing hundreds of murderous demons throughout the facilities. In general, the marine grits his teeth while shooting Hell's denizens, and when not actively shooting, is constantly on the look for more ammo and health. He seems to enjoy using weapons, as he shows a grin whenever he picks up a new one. He has a pet rabbit, Daisy, who he left behind on Earth when he was shipped to Mars. This bunny was killed by the invading demon hordes at some point before the end of the last episode of Doom, "Inferno". However, he got revenge by killing most of the demons that invaded Earth in the expansion episode, "Thy Flesh Consumed" of The Ultimate Doom, not to mention the sequel, Doom Il: Hell on Earth.
This character also makes an appearance in Quake III: Arena and its online browser based port Quake Live, under the imaginative name 'Doom'.
In Doom 2016, the Doomguy starts the game being released from a stone sarcophogi and quickly resumes killing demons during their latest invasion of Mars. Ingame lore reveals he is now known to the demons as "the Doom Slayer", a terrifying figure who brutally dispatches every demon he finds. After they conquered his home planet, he swore revenge on all of them. He promptly carved a bloody swath through Hell, slaying their greatest champions, and was only stopped by dropping a temple on him and imprisoning him in a casket. It is implied this is the same Doomguy that was in Doom 1, Doom 2, and Doom 64, although how exactly they fit into this canon isn't known.
"Doomguy" has no officially recognized name. Chapter text breaks in Doom and Doom II address the player directly as Doomguy ("YOU'VE NEVER HEARD OF ANYONE ESCAPING FROM HELL, BUT YOU'LL MAKE THE BASTARDS SORRY THEY EVER HEARD OF YOU!" etc.) In Doom 3, in-game characters never refer to the player character by name, only calling him 'marine'. In Doom 2016, he is called the "Doom Slayer", the "Hell Walker", and the "Unchained Predator" by the demons, while the UAC just nicknames "the Doom marine".
In the Doom series of novels by Dafydd ab Hugh and Brad Linaweaver, Doomguy is given the name Corporal Flynn "Fly" Taggart. This does not officially extend to the game universe though.
It is also suggested that Doomguy is a descendant of B.J. Blazkowicz, and is another continuation of that line (Commander Keen is acknowledged as B.J.'s grandson). However, his is never directly stated or implied within any of the Doom games.
He is seen dead and dismembered in Duke Nukem 3D. Duke Nukem says upon finding his corpse "Hmm, that's one doomed space marine".