Lets look at some of the possible clues in the text.
There are those references to your physical ability.
"You slam your fist in frustration onto the control board, leaving a dent." (Maunal page 2)Whether this is of significance or not depends largely on what material the control panel is made of. We have no information on this.
"It's funny, but you've always been the colony's trouble shooter. You're bigger and stronger, and a better shot. In games, you always scored the most points and looked the hero."(Manual page 2)In any large group of individuals we would expect to see a wide range in physical size and strength. As a security officer you would most likely have been selected on the basis of size and strength. Thus to have one outstanding physical person on the colony would not be too unusual.
Your physical ability is mentioned again in the Marathon II Preview. Durandal states:
Surviving such a fall, however, might be explained by the fact that you are wearing battle armor and are trained to take such falls unlike the colonists.Up the stairs from this location is a ventilation shaft that leads to the underground geothermal station. Because only you would survive the fall, you're going this mission solo.<What! About! Bob! (Terminal 1: 1st message)>
References to you being something other than human are made in the Marathon II Preview. In several terminals Durandal implies, through his selective use of words, that you are not human or at least should not be classed as human.
"...I dropped you and some humans..." <What! About! Bob! (Terminal 1: 1st message)>
"Ths area is not secure, and humans are still in combat." <What! About! Bob! (Terminal 3: 'Unfinished' message)>
"The humans are good fighters..." <BaseDestruction">BaseDestruction (Terminal 1: 1st message)>
There are also some less obvious clues.
How does Durandal know about your memories of your father and what he told you as a child?
"Your father told you as a child to always fight with honor, but to always fight."<Habe Quiddam (Terminal 1)>Are such memories implanted and accessible to others (e.g. replicants in Blade Runner)?
In Habe Quiddam, Durandal enquires about the origins of your capacities to kill, main and destroy and why you obey his every order:
Is this training or 'programming'?Strive for your next breath. Believe that with it you can do more than with the last one. Use your breath to power your capacities: capacity to kill, to maim, to destroy. And just where do your capacities come from? Why do you always go where I want and do what I say?<Habe Quiddam (Terminal 1)>
More intriguing are your unexplained memories:
"You can almost imagine the face of a wicked computer with its eyes wide and its lips folding out in a grotesque smile. A smile which reminds you of something from your past, but you can't remember exactly what it is." (Manual page 3)
"You pull out your pistol, and pound the switch to open the door. Oddly , this is familiar to you, as if it were from an old dream, but you can't exactly remember..." (Manual page 4)How is boarding ships in the blackness of space familar to you. Are these former memories of battle experience? Memories of the war:
"...between the Independent Asteroid Government of Icarus and its neighbor, the Republic of Thermopylae on the asteroid of Onicis 492." <Beware of Low-Flying Defense Drones... (Terminal 2)>perhaps? The war in which 'battleroids' were first used. A war in which 'battleroids' killed almost everyone and were later stored away in stasis chambers for safe keeping. Are you one of the ten cyborgs (battleroids?) brought on board the Marathon by Bernard Strauss?