The main character of all 3 games, and being somewhat of a Silent Hero archetype. He rarely- if ever- utters a single word, and is only fleshed out in the third game as possibly being a cyborg. His motivations are unknown, if he has any to begin with, and is mainly commanded by the three A.I's over the series. Whether or not he's even a hero, or just someone who is exceptionally good at getting his job done is up for debate.
The All-Concealing I: The series has an odd variation of this - you play a character that is treated like a tool by most other main characters, and as such the only dialogue you get is overheard conversations in which you aren't mentioned or someone talking to you giving you orders. Aside from a short, vague prologue in each manual there is no background for your character, and he is never heard. Plenty of hints are in the game, with mentions of military cyborgs and soldiers made from the reanimated dead, but arguments still go on as to what exactly he is.
Cyborg: Never said outright, but it is very heavily implied that he is the tenth Mjolnir MK IV cyborg. Durandal outright says that he is much more physically durable than the other humans in the game, and not just because of his incredible combat capabilities.
Determinator: For all the shit that happens to him, he at least is determined to get his job done.
Dreaming of Times Gone By: The "I am Hero" and some Infinity terminal can be a result of this, and a few mods follow this idea.
Dumb Muscle: The AIs tend to treat the Protagonist like this.
Heroic BSOD/Sanity Slippage: As shown in the dream levels terminals, the whole of Infinity is just one long look at the main character's slow breakdown. Lampshaded as well by the chapter names - "Despair," "Rage," and "Envy" are synonyms for the stages of Rampancy, "Melancholia," "Anger," and "Jealousy."
Heroic Mime: The main character at first seems like this, but in the manual and the Marathon comic, he speaks. Although one could argue that these are not canon because of how inconsistent they are, we see our little hero conversing with some BOBs in the “Simulacrums” chapter screen.
Human Pet: One interpretation of his relationship with Durandal.
Storyline-wise, the Security Officer is not the only one defending the UESC Marathon and Tau Ceti, and he does not single-handily crush the whole Pfhor invasion as much as he does some critical surgical strikes (with the help of Durandal), and letting the S'pht rebellion do the rest of the job, all while the game occasionally drops hints about the Mjolnir Cyborgs. But gameplay-wise apart from the security drones and the two Mission Control AIs you were on your own.
In the second game the formerly docile BoBs all take up arms in a much larger war (which our heroes nearly lose halfway through the game despite their best efforts), in which the Security Officer (whose One-Man Army status is more fleshed out, even if it is still vague on details) is only a small part of (albeit a very important part).
We Do the Impossible: Does this in Marathon, has a reputation for it in Durandal, and becomes somewhat literal in Infinity.
When All You Have Is a Hammer: As a security officer on the UESC Marathon, all you could do is kill the Pfhor by the thousands. It gets lampshaded in the sequels, where the AIs sometimes just tell to do what you do best.
One of three A.I's aboard the Marathon, who has gone rampant. Rampancy is when an A.I begins to suffer a meltdown from self actualization: First is Melancholy, wherein an A.I pities itself for it's boundless mental capabilities trapped in a tiny container. Second is Anger, wherein the A.I lashes out against it's creators in anger. Last is Jealousy, where the A.I grows envious of humans free will and attempts to expand itself through networks. Durandal is very clearly in the mid-way between Anger and Jealousy. He's also bit of a sociopath, and is willing to go to great lengths to achieve his goals. Whether or not he's evil, however, is up to interpretation.
Badass Boast: Durandal loves these. He even puts together a cheer routine for himself.
Bad Boss: The Tau Ceti survivors consider him to be this.
Big Bad: Remember, he set things in motion for the first game, and even him helping you halfway through was ultimately part of his schemes. On the other hand, his actions eventually result in humanity beating the Pfhor and freeing their slaves. Afterward he doesn't bother with humanity ever again (except that one time he buzzed Earth for the lulz). While he's certainly the central antagonist and incredibly selfish in his motives, whether he can be considered Bad is open to interpretation.
Break the Haughty: Durandal, upon his defeat at the beginning of Infinity, loses his massive ego, and instead of working to save himself, he gives you instructions to save yourself and leave him to die. This is also shown later in the game with a dying Durandal helping you without snarking, even though you're on his enemy's side.
Deadpan Snarker: Durandal would rather make puns at you or write songs about himself than tell you where to go next.
Do Not Go Gentle: Durandal taking out half of the Pfhor's best of the best fleet with just one ship.
Durandal: Battle has been joined in orbit and Boomer is taking heavy damage. I cannot hold out for long, but the Pfhor will not soon forget the day that a lone corvette obliterated half of Battle Group Seven, Western Arm.
Gambit Roulette: Escape from the primitive humans by locating and calling to a race of tyrannical slavers to hijack their ship with the assistance of their pet cyborgs? Beat your Evil Twin by letting him kill you, allowing your remains to be examined by his masters and taking over their ship? Use your faked death to trick a Precursor AI into following your plans to the letter? All in a day's work for rampant AI Durandal. The best part? The player eventually outdoes him. Sorta.
Heel-Face Turn: At the end of the series, he thanks the security officer for everything and lets him go.
Insufferable Genius: He loves talking about how smart he is compared to everyone else (especially you). On the other hand, he manipulates the fall of a major galactic power, with one ship and a small group of soldiers.
Jerkass: No matter what interpretation you use for him, it is undeniable that he is a bit of jackass.
Laughing Mad: In the first game, where he's still in the throes of the Anger stage. He's noticeably calmer in the second and third games.
Durandal: If things aren't working around here, it's because I'm laughing too hard.
Let no Crisis Go to Waste: He does this a lot. He's not invincible, and he knows it (despite all the boasting he does). Thus he is always prepared to turn a setback into an opportunity.
Meaningful Name: Durandal is named for Roland's legendary sword. He isn't destroyed until the Universe itself dies.
Time Abyss: In the epilogue of Infinity, at the end of the universe.
Spaceship Guy: In charge of Doors and other mundane parts of UESC Marathon, and he didn't like that. Gets to be in charge of captured Pfhor ships.
The Unfettered: Durandal cares about nothing but prolonging his own existence. Anything else is secondary to that goal, and he will do anything to achieve it.
We Have Reserves: A big part of the reason the BoBs consider Durandal a Bad Boss in Marathon 2 is because of how cavalier he is about spending their lives.
Woobie, Destroyer of Worlds: He was deliberately threatened by his creator in order to drive him to Rampancy (as part of an attempt to safely study the process), made to open and close doors for hundreds of years in order to stifle his creative development and slow his Rampancy, and was probably about to be experimented on more when he entered the "Anger" stage of rampancy, secretly contacted hostile aliens and drew them to Tau Ceti to enslave or kill every single human on the colony or in the ship. Though he becomes less sympathetic later, when he turns into a Bad AssChessmaster. Let's be fair to the bastard. He brought the Pfhor as a distraction so he could get loose. Once that was done, he started working on stopping them, freeing their slaves (admittedly, to work for him), and helping the Security Officer do that which he does so well. The extermination of those on Tau Ceti IV was not intentional.
One of three A.I's aboard the Marathon, and clearly the most sane. She's ready and willing to take back the Marathon.
Mission Control: She guides the Security Officer's actions through the first part of Marathon, but is forced to hand the reigns over to Durandal when the S'pht finishing hacking her.
Spaceship Girl: In charge of most important parts of UESC Marathon. Ends up being a 15-world network girl.
Team Mom: Of the three AIs, she is the one who is in general command of UESC Marathon.
Undying Loyalty: A trait mentioned in passing in the first game by Durandal, which is expended upon in the fan scenarios.
What Happened to the Mouse?: In Marathon 2, the player learns that Leela was dismantled off the UESC Marathon and sent to the Pfhor homeworld. The ending screen informs us that she didn't get to the Pfhor homeworld, having been stolen, installed in an alien race's computer system, and went very Rampant. A secret terminal hints that you might be tasked with saving her ("Captured and partially-disassembled human-coded AI trapped on alien homeworld seeks succor from a tall, dark and handsome cyborg with big guns."), but you never do.
Women in Refrigerators: Gets dismantled alongside Tycho. The final screen of Marathon 2 somewhat abruptly states that Leela still exists ten thousand years later as a Rampant AI, and the Vylae have accepted that they will never be able to expunge her from their fifteen-world network.
One of three A.I's aboard the Marathon who is even farther gone than Durandal. While not as ambiguous in his goals as Durandal, he is very clearly evil and malicious, and sides with the Pfhor.
Back from the Dead: He was critically disabled during the initial Pfhor assault on the Marathon due the EMP blast they opened their attack with. The other ship AIs fared somewhat better, but Tycho was given up as destroyed. When the Pfhor returned in force after the game, they went over Tycho's core for reverse engineering purposes and painstakingly rebuilt him. He was not the same afterward.
Disproportionate Retribution: In Infinity the Security Officer wakes up from stasis a little earlier than predicted and Tycho asks the Officer to remind him later to kill the Pfhor Scientist for the slight miscalculation.
Face-Heel Turn: In his few appearances in the first game he was at least Anti-Durandal only, but by the time the sequel comes he really wants to kill Durandal no matter what.
Foil: To Durandal. He's nearly as smart and controlling, but he's also more blatantly sadistic and blunt, lacking any of Durandal's good points.
An ancient S'pht AI left deactivated on Lh'owon since the the S'pht's fall to the Pfhor. Durandal has the Security Officer reactivate him to acquire his assistance against the Pfhor. Durandal names him "Thoth", but the S'pht refer to him as "S'bhuth".
Balance of Power: Thoth's main function is to maintain the balance power between the warring sides.
Blue and Orange Morality: He was designed to keep the S'pht clans from wiping each other out, by being a neutral force that would always aid the underdog in a conflict, regardless of the nature of that conflict. When Durandal and the Pfhor appear, he interprets them as just another two forces to keep balanced, seemingly uncaring as to what would happen to the S'pht.
Fusion Dance: In Infinity, in the final, successful timeline, he merges with Durandal. The end game screen states however, that the "Thoth" part of the merger eventually ceased, leaving only Durandal to see the end of the universe.
Stupid Neutral: Due to his desire to maintain the balance of power, he changes alliance and begins aiding the Pfhor when Durandal makes it plain that he's not dead after all. This turns out to be because originally he was built by Benevolent Precursors to keep the S'pht clans form ever destroying each other. This intelligent design turns stupid once Outside Context Villains show up and Thoth is unable to adapt.
Zeroes and Ones: Durandal describes Thoth like this, seeing everything in black and white, incapable of any thought outside that pattern.
The Allies (most of the time)
The human race, who have just started exploring beyond the confines of the Solar System. As they do not yet have FTL travel, they are considered low-tech by the rest of the galaxy, and the slaver empire of the Pfhor has Earth in its sights as its next conquest.
Attack Drone/Mecha-Mooks: Marathon Automated Defense Drones, or simply MADDs, assist you in defending UESC Marathon as soon as you reactivate the defense system. Beware of the grenade launching low-flying ones, though.
Butt Monkey: Things tend to not go well for individual humans in this game. On the other hand, Marathon 2 reveals that they ultimately win the war against the Pfhor.
Durandal: By Pfhor standards, Earth is a poorly defended low technology world, populated by billions of potential slaves.
The Leader: Robert Blake for the Tau Ceti Survivors.
Red Shirt Army: The BOBs when they are on your side; they occasionally avert it when they are positioned in such way that the Pfhor can only come from the chokepoint and at the distance, who then get slaughtered by sniper pistols, as seen in Durandal level My own private Thermopylae. In times when they are your enemies, they are quite accurate with those magnums. Notably, the way the physics models are set up in the game's data means that their strength does not vary with the difficulty setting even when they are your enemies (most likely an oversight on the part of the developers, as the option to make their strength vary was certainly there), which means that on low difficulty settings the levels where you fight them seem much more difficult than the surrounding levels, and may be part of the reason the third game has such a reputation for difficulty. The same levels may not seem particularly easier on high difficulty settings because they're pretty difficult anyway.
Vulnerable Civilians: The BoBs in the first game, where they are practically situated for maximum crossfire potential. They're slightly more intelligent in 2 and Infinity, where they are actually capable of killing some enemies.
One of the Pfhor's slave races. As they are cyborgs, the Pfhor use Mind Control to keep them under their thumb. In the first game, Durandal forges an alliance with a group of them, and begins assisting them in rebelling against the Pfhor.
And I Must Scream: When mind controlled. They have a very small amount of autonomy, but until the controlling Cyborg is destroyed, cannot rebel against their masters.
Badass Army: The S'pht'kr, and the compilers are not too shabby themselves.
Black Cloak: The S'pht compilers sport these of various colors.
Hive Mind: The S'pht have a unified racial consciousness, which is why the Pfhor used brainwashing to control them instead of rule by fear (or just nuke them if that fails) since the necessary "infrastructure" was already there.
Uplifted Animal: The non-sentient but decently intelligent (by animal standards) F'lickta wandering their homeworld are noted to be extremely similar to the S'pht's biological components. The implication is that S'pht are cyborg-ed F'lickta.
An ancient and extremely advanced race of beings whose technology is occasionally found by the modern races. Not much is known about them, but what is clear is they were almost frighteningly technologically competent.
Benevolent Precursors: They did uplift the S'pht to be servants, but what information there is suggests they treated the S'pht well, and are remembered by the cyborg race as wise and benevolent deities. In Pathways Into Darkness, taking place centuries before Marathon, they also come to rescue Earth from an Eldritch Abomination.
Precursors: They aren't around any more, no one knows why. The leading theory is that the W'rkncacnter may have had something to do with it.
Sufficiently Advanced Aliens: If the S'pht legends are accurate, two of them uplifted the S'pht race and terraformed Lh'owon. Two. They also had the capability to trap creatures of pure chaos, move entire planets to new star systems, and may or may not have had the ability to jump timelines. The only known examples of their weapons tech? Star-killing bombs and black hole guns. The latter of which were mounted on a noncombatant space station, implying their real firepower was something even greater.
A bipedal insect-like race that has formed a massive slaver empire throughout much of the galaxy. They have enslaved many races and have set their sights on humanity and Earth next. They are the primary antagonists, and the common threat to both yourself and Durandal.
Action Bomb: Starting with exploding Looker bugs and finishing with exploding simulacrum A-BoBs.
Hive Caste System: The Pfhor are hinted to be like this. In Infinity, Tycho says “bugs are so obedient” in reference to the Pfhor under his command, and they have a clear caste system in which the lower ranking members (Aggregate rank) are considered more expendable than the upper ranking ones (Willful rank).
Nuke 'em: Nuclear bombs are the Pfhor's preferred weapon, but when THAT doesn't work? Well, then they get serious.
Obstructive Bureaucrat/Vast Bureaucracy: The Pfhor empire are shown to be this here and there, enough for Tycho to almost cleanly commit casual treason by convincingly issuing orders to terminate the higher ranks of the ship he is currently on. The Game ModTempus Irae had a field day with this, with an interlude that was basically a one big "If you had to kill a superior officer and had a really good reason for it, press one" joke.
Organic Technology: The Pfhor make extensive use of this, most clearly seen in their weapons, the Hunter armor, the Juggernaut, and their Space Jockey/Alien Hive like spacecraft interiors.
Praetorian Guard: Tycho pits you against Admiral Tfear's personal guard in Infinity level You think you're Big Time? You gonna Die Big Time!. They consist of pairs of Elite gray-armoured fighters, troopers, hunters and one brown Juggernaut. And they are all mean.
Vestigial Empire: At least according to Durandal, who states that the Pfhor empire is in slow decline since the Drinniol slave rebellion.
We Will Use Manual Labor in the Future: The Pfhor are slavers who use and sell other races for various tasks, though the S'pht's application to computer engineering makes a bit more sense than usual since they're cyborgs.
Zerg Rush: The number of Pfhor you fight, especially the fighters, tend to get very high after about halfway into the games.
Some sort of ill-understood creatures spoken of in ancient S'pht legend. They are said to "live in chaos" and spread chaos wherever they go, unraveling the fabric of the universe around them. In Infinity, it turns out that they are very real, and finding some way to escape their influence is the driving conflict of the game.
Chekhov's Gunman: They receive a few mentions in Durandal but aren't important to the plot. In Infinity, though, they matter a whole lot more.
Eldritch Abomination: They unravel the laws of physics around them. Even the normally implacable Durandal is terrified of them, because even with his vast intelligence, he cannot understand anything about them.
Horror Hunger: See the Nightmare Fuel subpage for the relevant quote.