In the original Doom, he's a nameless marine sent to do grunt work on Mars after an incident over not wanting to kill innocent people on Earth causes him to lash out against his commanding officer. During his dull-as-dirt assignment, it seems the scientists of the Union Aerospace Corporation (UAC) are conducting experiments with teleporters when everything falls to pieces. One of Mars' moons and a UAC outpost, Deimos, completely vanishes into the ether while another, Phobos, is immediately overrun by The Legions of Hell. Everyone is either slaughtered or converted into minions of the invaders. The marine dispatches to Phobos, where he proceeds to clean up the situation as brutally as possible. During his one-man war against the forces of Hell, he discovers what became of Deimos (ended up floating above Hell itself) and then treks down to the surface of Hell to finish the job.
When the sequel, Doom II: Hell on Earth, rolls around, the marine had just finished ripping Hell a new one when a distress signal alerts him that the armies that invaded the Mars moon bases have now breached their way to Earth. The humans who are able to escape the genocide decide their only recourse is to escape the planet but their exit route is barricaded by the invaders and obstructed by a force field they set up. The marine is called to break the line of defense and deactivate the force field on behalf of the other humans so they may escape, which he does and remains as the last human on Earth. However, further communication from the escaped humans reveals that the epicenter of the invasion is still on Earth, allowing the marine to finally push back against the invaders. After fighting their forces he decides to cross back into Hell yet again, this time with the intent of making sure they can never invade anywhere ever again. After this trek through Hell, he manages to essentially destroy it, literally leaving no place for the damned to go once they die. Come Doom 64, Earth is finally mopping up from the attacks from the first two games, when something resurrects all the demons on Phobos and Deimos, prompting another assault that ends in the marine storming Hell a third time and apparently wiping demons off the face of Hell completely by killing the Mother Demon.
Charles Atlas Superpower: There's nothing supernatural behind his Berserk Punch and ability to withstand direct hits from a rocket launcher.
Like a Badass out of Hell: Three times. In the first game he ends up in Hell after discovering where Deimos had teleported to and wanting to teach the demons a lesson on messing with him. He makes it out in one piece but in the second game, he voluntarily returns to Hell and completely wrecks the joint, to the point that he is left to wonder what would become of the evil men in the world when they die. In Doom 64 he kills the Mother Demon and apparently exterminates the demons completely.
One-Man Army: It's you and every demon that the depths of Hell can throw at you, get to it.
Chronic Hero Syndrome: After returning from Hell at the end of the first game, he volunteers to lead the strike force to recapture Earth's last spaceport and evacuate the last surviving humans. Then after that he volunteers to go back into Hell to reverse the invasion.
No Official Name Given: While he has plenty of fanon names (and a couple arguably canonical), the Doomguy really has no official, cemented name. Romero stated that he wanted the players to feel like THEY were the marine and to invent their own personality.
I Have Many Names: Current fanon candidates so far are Extermination Warriornote His Chinese name, also the Chinese name of the Doom game., Doom Marinenote As he is the marine of Doom., Buddy Dacotenote The only named character from the original script; he was a support character who, like the finalized Doom marine, died at the end of episode 1., Flynn Taggartnote The name of the character in the Doom novels., Stan Blazkowicznote The name given in the Doom II RPG for iPhone, descendant of Wolfenstein/B.J. Blazkowicz., John "Reaper" Grimmnote The name from the Doom movie., Doomnote The name given to him in Quake III Arena, likely an alias., John Kane note The name given in the Doom 3 novel., and John Stalvernnote From DOOM: Repercussions of Evil, possibly..
I Choose to Stay: At the end of Doom 64, Doomguy decides to stay behind in Hell to make sure he's killed off the demons for good.
These were the soldiers on Mars (and then Earth) who were "conscripted" into the armies of Hell as front line defenders. They are essentially zombies and increase progressively in toughness (Grunts go down in a few shots while Commandos are armed with a chaingun and will take some punishment).
When Hell first breaks loose in Doom 3, the majority of the civilian population are quickly possessed by Demonic spirits and transformed into Zombies. Alone, they're very little threat, due to their lack of agility, but they have a nasty habit of popping up from behind corners.
Zombie Gait: With a few exceptions, most shamble slowly towards you.
Stands for "Zombie Security", the basic security troops of Mars are also quickly turned to Hell by the Demonic spirits. They avert their civilian counterpart's Zombie Gait and attack with whatever weapons they had while alive.
Short Range Shotgun: Averted: the shotgun-wielding variety has a narrow spread on his weapon, allowing him to shoot from far distances.
Zombie Gait: Averted: these zombies retain their full mobility from life.
Hell's own soldiers. Monsters who are capable of throwing fireballs, Imps are the lowest level of demon that lead the charge for the forces that follow.
Hulking brutes that are tinted pink. These guys get in your face as they have no ranged attack but will do some damage. Spectres are versions of Demons that only appear as hazy outlines when encountered. Doom 3 gives an extensive redesign to Demons, but effectively they're the same.
Disembodied horned skulls that are on fire. They can spawn from other enemies and simply fly into you as their only means of attack.
Expy: In Doom 3 the Lost Soul was redesigned as a floating, hairless human head with a rocket attachment at the back and wide-split jaws. In Resurrection of Evil they brought back the original "flaming horned skull" design and called it the Forgotten One.
Hell's elite. They are hoofed beings that walk upright and have a ram-like head, basically giant Satyrs. The Hell Knights are brown while the Barons are pink and tougher. They throw bolts of green fire and can slash with their hands. The Barons first appear at the end of the first episode of Doom as the final boss.
Alien Blood: They also bleed red when shot, but their death sprites clearly have them bleeding green.
Though the Barons bleed red in Doom 64.
Contractual Boss Immunity: Barons and Hell Knights are immune to their own and each other's fireballs (though this is changed in Doom 64.)
I'm a Humanitarian: His lair in the original DOOM features several half-eaten Barons of Hell strung up on the walls, the idea being to make you worried about what could possibly be dangerous enough to look at the Barons —who up 'til now were your most prominent Demonic Spider enemy— and think "Dinner!"
Lightning Bruiser: He's the fastest-moving monster in the game apart from the Archvile and the Lost Soul when it's charging, has 4000 HP and an immunity to splash damage, and can easily splatter an unwary player who's a Glass Cannon by comparison.
Mascot Mook: Not anywhere near as popular as the Cacodemon, but still the second most recognizable enemy in the game.
One-Hit Kill: Gains the ability to stomp on the player for an instant kill in Doom 64 (as well as the Brutal Doom mod.)
The final boss of Doom 64. A demon with the ability to resurrect other demons much like the Arch-Vile (only making them stronger than before.) Single-handedly rebuilds the demon army, forcing the marine to put them down another time.
Gameplay and Story Segregation: Apparently all she needs to do is pass by a demon and it comes back to life. You fight her in a room likely covered in dead demons (unless you use the Demon Artifacts to seal off the portals before any come through) and they all stay dead.
Monster Is a Mommy: Possibly. Earlier in the game the player hears "They took my baby" said by a voice similar to the Vagary's.
Also, given that the Vagary's boss fight takes place at the end of the Alpha Labs, which is literally crawling with the other spider-based enemies, the Trites, there is a popular theory among the fandom that the Trites have established an insect-like colony within the Alpha Labs with the Vagary as their queen.
Guardian of Hell
A massive blind beast that chases the player through the bowels of Hell.
A trio of elite demons sent to retrieve The Artifact and spearhead the second invasion of the Mars base in Resurrection of Evil. The team consists of the Helltime Hunter, Berserk Hunter, and Invulnerability Hunter, which you fight in that order.