The birth of the Spartan in Halo is the birth of a hero, as well as a hero’s journey.
Master Chief is a man that is bred for war. Kidnapped at a very young age and raised by Oni to be an elite soldier against the insurrection, war is all he has ever known. Before he can be properly deployed against the insurrection, however, humanity finds itself thrown into a war against a ruthless alien force called The Covenant. This is what we would like to call the call to adventure, expecting shortly after to see the mentor, in which case we can rightfully conclude that Cortana fits the role. The threshold of the story is psychological determinism, or undoing the hard wiring imposed on him by Oni. What we expect to see from a spartan is a cold soldier relentlessly focused on defeating the foes that would oppose him, and though we see this in Chief, we also see a man who is torn by his existence, and the deep psychological need for companionship. So Chief fights through 3 entire games of trials and temptations and then finds himself in the middle of his death and rebirth. He encounters The Librarian, who reveals to him that his existence was set in motion by the forerunners, and that the humans must take the mantle of responsibility for the universe. This sets in motion a journey of truth and redemption for Jon, and though we can not yet conclude how 343 is going to end his story, I think it stands to reason that through his redemption, he’ll return to where he came from as a changed man, and one who can sew his own fate.
“Hero’s aren’t born, they’re created.”