Also known as "Doomguy". 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 kicking 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.
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: Twice. 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.
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.
Guest Skater: Doomguy is a secret character in the PC version of Tony Hawks Pro Skater 3, due Gearbox Software being the one who ported the Windows version of the game.
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.
Sure, Why Not?: Out of all the nicknames and made up names, Doomguy seems to be the one which the creators have at least accepted as a Half-Canon Name, as it got numerous nods and actually appeared as the Marine's name in THPS3.
Enemies
Former Human Grunt, Sergeant, and Commando
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 Doome 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.
Glass Cannon: They can deal surprising damage (especially the fat ones) but their slow speed and lack of durability cause them to be minimal threats unless they sneak up on you.
Goddamn Bats: They can be annoying due to their tendency to appear out of nowhere and surprising quietness compared to other enemies.
Zombie Gait: With a few exceptions, most shamble slowly towards you.
Z-Sec
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.
Imp
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.
Glass Cannon: They can easily kill you, but they explode after 1-2 shots from the shotgun.
Fragile Speedster: But be careful with your aim; buggers can be tough to hit.
Your Skull A 'Splode After being killed. The explosion is merely an effect, though, and won't affect anything near the Lost Soul.
Cacodemon
Floating blobs of flesh with a single eye, horns, and a nasty maw. Doom 3 redesigned the Cacodemons to look more like Pain Elementals, but they're similar to the older variety in gameplay.
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.
Dual Boss: A pair of Barons at the end of Episode 1.
King Mook: In Doom 3, Hell Knights are absolutely massive.
Palette Swap: Hell Knight is one. Oddly, he uses a separate set of sprites.
Pain Elemental
Another floating blob of flesh with a single eye and unpleasant mouth. These guys are brown (unlike Cacodemons, which are red). They can create Lost Souls, which is how they attack.
A being that can revive fallen monsters. Also attacks the player with flames that can erupt underneath the target. This monster is exclusive to Doom II.
Kill It with Fire: How he deals with you. The fire doesn't hurt you, it only marks the place for the explosion he'll cause.
Moral Myopia: Screams "why?!" upon death because he is Hell's "evil healer". He doesn't understand why anyone would want to kill him since he's only doing good by resurrecting his fellow demons. And then burning you to death.
Bosses (original series)
Spider Mastermind
The final boss of episode 3 of Doom. A gigantic version of the Arachnotrons, it too is a creature mounted upon four titanic metal legs and a chaingun for defense.
Brain in a Jar: And by "jar" we mean a cybernetic weapons platform.
Contractual Boss Immunity: To splash damage. This effectively gives it nearly 70 percent resistance to rockets.
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.
Warmup Boss: The Helltime Hunter exists mostly to teach the player how to use the Grabber.
Dr. Bertruger & Maledict
The new leader of Hell, fought at the end of Doom 3: Resurrection of Evil. A giant pterodactyl-like or gargoyle-like demon that spews fire and summons meteor showers.
Be Careful What You Wish For: In the final cutscene, he demands that the marine gives him the Artifact. The marine shoves the Artifact down the Maledict's throat, killing him almost instantly.
Contractual Boss Immunity: Inverted; his attacks cut through your invulnerability if you try to use the Artifact.
Heads I Win, Tails You Lose: After his health is depleted, the fight immediately switches to a cutscene where he knocks the player's marine on his ass.
