The Security Officer, also known as Mjolnir Recon 54, is the player character and main protagonist of the Marathon Trilogy
His background not truly known, The Officer was a security guard working on the Marathon space colony. However, an alien race called the Pfhor attacked the colony, forcing The Officer to defend it with the assistance of Leela, one of the ships 3 AIs, the other two being Durandal and Tycho. As he did, he discovered more about the ships history, as well as the context of the current issue, and how it was much more than it apparently seemed. Apparently, Durandal had gone "Rampant", which is when an AI gains full sentience, and summoned the Pfhor so that he could steal their ships, which posessed space-folding technology that the crazed AI desired for the purpose of escaping the Big Crunch. However, Leela was shut down by the Pfhor hackers, forcing the Officer to form an uneasy alliance with the unstable Durandal. Eventually, the Officer was able to free the Pfhors enslaved S'pht from their cybernetic brainwashing, causing a massive revolt that crippled the invasion.
After the events of the first game, the Officer was kidnapped by Durandal and put into stasis for 17 years. He awakened on the planet Lh'owon, homeworld of the S'pht, where the humans freed from the Pfhor were waging a war against the slavers. He then began his journey to go find more advanced technology that Durandal planned on using for unspecified purposes. During his task, he ended up having to kill Durandal before the Pfhor could capture him. However, he discovered this was part of Durandals plan to awaken an old alien AI called Thoth, who could be used to turn the tide of the war. Eventually, Durandal revealed he faked his own death, defeated the remaining Pfhor ships, and had the Officer defeat the best fighters in the fleet just for added humiliation.
After that, the Pfhor used the Triah Xeem, a weapon designed to force a star into early nova. However, this resulted in a chaotic being called the W'rkncacnter, which was apparently imprisoned in Lh'owon's sun, being unleashed. It began to devour reality, and The Officer started jumping across multiple timelines to find a way to stop its chaos, while also learning more about his own history and identity.
From what little we know abut his overall disposition, The Officer seems to be a dutiful soldier, albiet a somewhat detached one. He tried to the best of his abilities to save the population of the Marathon, as well as the Tau Ceti colony it overlooked. The Security Officer is willing to do anything in order to acheive long-term victory over his foes, even if it involves fighting those he considers allies.
The Cyborg's mind and personality are mostly left up to the player to speculate on. Although Durandal describes him as "a magnificent killing machine" and asks him if being allowed to kill more Pfhor will "make you happy," the AI is more likely being malicious than accurate. While the Cyborg does not seem to be a mindless psychopath, he is clearly comfortable enduring and quite capable of carrying out violent acts on a scale unimaginable to any normal person. However, in the third game of the series, he seems to lose any sense of morality he may previously have possessed, working for the indisputably evil Tycho and killing BOBs on Tycho's orders in a desperate attempt to keep the W'rkncacnter trapped in Lh'owon's sun. In Marathon Infinity, we are given greater access to his mind and feelings. He seems to recognize some sort of guilt or weariness for the atrocities he has committed, and appears to believe that he has been forced to do what he has done . Ironically, this seems accurate. All three games consist chiefly of doing what various people tell him to do - only once does he act of his own initiative, and this is in the Marathon manual, which is not an entirely reliable source.