A Spartan would be great. Spartans are associated with the UNSC, which makes them immediate soldiers for the Martial alliance (Unless one has decided to go rogue). Spartans go through the same training and history as they did through the game, the only difference being that Spartans work to protect Mars, not Earth. 

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Codex Update: 

2525: Project Olympus is put into place by the Mars Alliance. After years of research thanks to the ORION project, average UNSC soldiers are now augmented with 100% survival rate. These augmentations fall short of the ones SPARTANs received, but they still are very helpful. They include, increased mussle mass, increased lung capacity, a decrease in reflex time by 30% and enchanced senses. Marines now wear Mark V SPI armor, now upgraded to enhance speed and strength, and are equipped with personal shielding, although it still falls short of MJOLNIR's capabilities. ODSTs have had their regular armor upgraded with personal shielding, improved alloys, and an improved HUD. (HALO)

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Also, if you look at the under the "military strengths" spoiler in my second post, you'll see there are several career options within the Martian alliance. A Spartan, in this case, would probably be a security guard for some of the higher ranking officials in the city.