- Sergeant Jack Marine agreed to lead the excavation team in order to discover what happened to his brother, the now-retired Corporal John Marine, who hasn't said a word since he returned from Mars.
- Maybe the first Doomguy? He gave aid from... An Alternate Universe!
- Every time the first Doomguy missed in the first two games, he wasn't actually missing his targets. The shots crossed dimensional boundaries, went to the Doom 3 reality, and killed the demons there. It's the only reason he "missed" at all.
- Considering that it can be accurately described as a "sentient weapon", maybe it was the Soul Cube (or specifically, the souls that helped create it/inhabited it).
- Maybe it was Alma! (see below)
- As there is nothing to say this isn't possible, maybe it was an immortal, even more badass, Willow.
- Maybe the first Doomguy? He gave aid from... An Alternate Universe!
- The voice also unlocks a hidden closet full of ammo for you after spooking you a bit, which suggests it's on your side and opposed to the Hell Forces. Perhaps Alma doesn't like competition.
- Heck, that even gives a reason for the marine's edge. Pariah gene, anyone?
- There are two missing Primarchs, given the amount of daemon ass that gets kicked...
In the latter levels of the game, there are several theories that suggest the Martians who survived the demonic invasion and didn't sacrifice themselves to create the Soul Cube fled to Earth. The Marine could very easily be a descendant of those refugees, for a couple reasons shown in the course of the game:
- Betruger's interest: Betruger really couldn't care less about most of the marines in the complex, sending swarms of lesser demons (Imps, Wraiths, etc.) to finish them off. But note that a) More powerful enemies appear to challenge the Marine on his journey through the facility, and b) the areas of the facility that (literally) go to Hell as soon as you get near them. It's possible that Betruger somehow knows (via the demon he hosts, maybe?) that the Marine isn't a mere human grunt, thus he tries to slow him down/finish him off at every opportunity.
- The Soul Cube: When you reach the part of the Delta Labs where they were analyzing the Soul Cube, you can hear the scientist talking about it in a PDA audio log. In that log, he says they've deciphered what the markings on its exterior say, but they cannot get it to activate in any manner. Yet, when the Marine reaches the Guardian in Hell, it's activated (with spinning blades and everything), and it's speaking. This would imply that it only activates when a) an invasion by the demonic forces has begun, and b) it senses that there is an individual nearby who is Martian/of Martian heritage. It would give a new twist on why Betruger took the Soul Cube into Hell in the first place. Not only would the demons have possession of the most powerful weapon that could be used against them, but the high chance that the Marine would be killed while fighting through Hell could guarantee that no one could recover it until it was too late (ie, the demons have retaken Earth).
- And the Hellportal is a prototype of the Stargate. Think about it the Stargate Universe gate spins while dialing, the Hellportal spins the whole time it's on, perhaps to keep the established connection with Hell active. And then once the Ancients found a better way to make portals, they abondoned the Hellportal for the Stargate.
- He has sunken in eyes and one of his eyes are bright fuckin' white, plus he's power hungry, pale-skinned, and want to make Hell be everywhere and kills anyone who gets in his way.
Depending on which continuity you go with—with the series either ending with Wolfenstein 2009 or with the alternate history of The New Order—B.J. Blazkowicz volunteered to use the technology recovered from Castle Wolfenstein to have himself frozen in cryogenic stasis, to be awakened if the technology should ever be used to threaten mankind again. Centuries later he was found in the castle ruins by Union Aerospace forces, but the degradation of his capsule caused problems with his revival, which cost him his memories. The executives of the UAC—having known of Blazkowicz and his history—immediately saw the potential use for him as a one-man army, and he was trained in the elite military under the name John Kane. One single act of defiance led to his transfer to Mars City, something Betruger didn't expect...
Dr. Malcolm Betruger not only bears a bit of resemblance to Wilhelm "Deathshead" Strasse—bald old scientist with two different eyes—he also shares Strasse's brilliant mind and keen affinity for research and development. Betruger is also a German-sounding name. When Betruger first entered the portal to Hell found in the ruins on Mars, he came face to face with the vengeful spirit of "Deathshead", who the Icon of Sin had given command of its demonic armies for its invasion of the mortal realm. "Deathshead" possessed Betruger and was fully reborn in his body, sent to open the gates of Hell and unleash the invasion. The one thing "Deathshead" hadn't counted on was his old enemy—the amnesiac B.J. Blazkowicz now known as John Kane—being transferred to Mars.
If you're going with the Doomguy's name used in the Doom RPG, then somewhere along the line B.J. possibly got his memories back and met a woman who he started a family with. This would eventually lead to Stan Blazkowicz, the Doomguy, who would continue his predecessor's battle against the forces of Hell.
There never was a woman with a baby, or at least one that the player encountered. The signs of one, including the ammo stash reward, are traps for Human males hard-wired culturally to hear this sweet-voiced young mother call on them and respond with instant go-getting. But Doomguy was different. Very willing to help, but not rushing in without checking the terrain. Now, this isn't putting harming a child past these forces - its possible the demon pregnant with a mancubus was once a Human mother and infant, and a horrific bloated corpse on the wall near the final battle could be a late infant - but this particular set-up—was a set-up, and once Doomguy took it slow (as gameplay almost requires), the trap's value was gone.
As Eternal deliberately confirmed, the classic 1/2/64 Doomguy and the modern 2016/Eternal Doom Slayer are one and the same, though is also only implied to be the same ones from 3 as well. A reasonable explanation would have to be: 3 and its expansions all take place between 2 and 64, as humankind starts to slowly return to a healing Earth. While he is already a marine in the main branch of the UAC, he could have enlisted in another, more scientific/experimental branch, hence the different armoring and aesthetics. And as for why he became an engineer, then a Bravo Marine? Of course he would take these different occupations - they're just bridgeways to killing bigger hordes of demons. He may not be a Blood Knight yet, but more of a Properly Paranoid individual who is willing to change his identity, even his face, just to get demons away from Earth. Plus of course, as for the reasons why all the demons look different: it's a different strand of demonkind, a different race, almost. This is why he simply fought to survive in this game - these aren't the oh so familiar bastards he shot up before. There is also a better explanation...
The Doomguy in 3 is an Action Survivor. Not an unstoppable badass yet, and just trying to fight for his life against older, predating versions of the varying demons he would fight for ages. However, he finds himself tempted to return again and again, finding himself obsessed with killing demons as his selfless cause - to ensure no one else deals with demonkind but him - starts to build, over the course of the expansions. In Resurrection of Evil, he is already looking weathered and older due to the trauma of the previous incident, and now already has the proper furious, rampaging demeanor he would carry for the series - but he's working on the morals, hence him wiping out his squadnote . Upon killing Maledict, the pre-Face–Heel Turn Makyrs pull him from the brink of death, and so he joins the Bravo Marines, becoming The Faceless for the first time and more or less uses the occupation as an opportunity to just get another shot at the demons, which he does at least twice.
Returning to the UAC, he is promoted to the head division of marines as the events of all the other games unfold, finally the Doomguy he is known as, killing the strand of demons. Over 120+ Earth years, he kills demons across four games (Doom, Doom 2, Final Doom, Doom 64), kills more through a lot of fan .wads (which for us would equal to just a few decades, but for Doomguy and Hell, it's centuries), meets the Night Sentinels, gets his Divinity powers, kills more demons over four ages before being imprisoned (basically the fan .wad thing but four times longer, so this and his imprisonment equates to millennia in Hell's time), kills more demons across two more games (DOOM, DOOM Eternal) and then finally kills Satan (The Ancient Gods) - and to think this was some doofus marine once upon a time in a space station. The only continuity error left would be the notes left by Dr. Richardson, which were made 12 years (2163) after the events in Eternal (2151), though this can just be seen as another case as another terminology, in this case it might've just been 2163 years since the last sighting of non-demonic greater beings such as Maykrs.
And one more thing...
But think about it - Eternal has hinted that Hell has some Timey-Wimey Ball factored into it; you could say that through Hell's chaotic time zoning, Doomguy ripped and tore so much and so hard that he warped into the distant, very distant past; stole some Martian armor; and fought the Mother Demon again with the Soul Cube, but of course didn't slay her; otherwise Doom 64 wouldn't happen and a paradox could have stricken the universe.
Just like with 2016, they tried sealing him away with a sarcophagus...and it didn't work, as he not only broke out immediately, but also killed his way back into the correct time to continue his four-age rampage.
