Doomguy is the name given by fans to the anonymous space marine that the player controls throughout every game in the Doom series.
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. Creator John Romero has stated that "there was never a name for the DOOM marine because it's supposed to be YOU."
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.
Doomguy appears in many of the sequels although it is unclear if there are of the same person. In Doom (2016), the Doomslayer is implied to be the same Marine from Doom, Doom II, and Doom 64, after sealing himself inside Hell for an unspecified amount of time at the end of Doom 64.
In Wolfenstein RPG, it is hinted that the Marine is a descendant of William "B.J." Blazkowicz. In a reference to the Marine's confrontation with the Cyberdemon, when Blazkowicz defeats the "Harbinger of Doom", the creature states that he will return in the future to confront his descendants.
DOOM & DOOM II
Next to nothing is known about him specifically, apart from the incident that got his ass to Mars: not only had he refused to follow a direct order to fire on innocent civilians, but he also 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.
Hell tries once again to reach our universe in Doom 64 under the leadership of Mother Demon. Thankfully, Doomguy manages to re-enter Hell and kills her. After this, the Marine accepts his fate of eternal confrontation with demons and decides to close the gateway between Hell and our universe from inside, thereby staying in Hell forever to prevent any further invasions once and for all.
In Doom 3, Doomguy is simply referred to as "the Marine." His appearance is more human with the lack of full-body armor and helmet, making him more vulnerable looking than in previous incarnations. The Marine is the newest member to be stationed at the UAC station on Mars. He is immediately sent to find a missing scientist in one of the facilities around the same time the UAC has accidentally open the portal to Hell, unleashing mayhem on Mars.
With him miraculously being one of the few who did get turned into a possessed soldier, the Marine sets off to defeat the Hellish horde and eliminate Dr. Betruger, Head Scientist and the man responsible for the disaster. After defeating the Cyberdemon in Hell, the Marine safely returns to the UAC station alive when he is rescued by the reinforcements from Earth.
DOOM 3: Resurrection of Evil
The Marine in this expansion pack is a different protagonist who is also nameless and voiceless. He is a combat engineer who gets sent to investigate the now derelict UAC Station on Mars, exactly 2 years after the events in DOOM 3. The marine arrives in Hell and battles his way to Betruger, who has become the dragon-like Maledict. After some fighting, Betruger bites the marine, but before he can eat him, the marine shoves the Hellstone down Betruger's throat, causing him to dematerialize. Only his skull remains. The game ends with a bright white light, followed by supporting character McNeil's voice saying, "Marine?... Welcome home."
In Doom 2016, the Doomguy starts the game being released from a stone sarcophagus and quickly resumes killing demons during their latest invasion of Mars. In-game 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.
He is seen dead and dismembered in Duke Nukem 3D. Duke Nukem says upon finding his corpse "Hmm, that's one doomed space marine".
This character also makes an appearance in Quake III: Arena and its online browser-based port Quake Live, under the imaginative name 'Doom'.