The Master Chief
The Master Chief was the sole Spartan to leave the Reach system aboard the Pillar of Autumn (with the exception of Linda-058 in cryo). For all he knew, he was the last Spartan alive. All of humanity depended on him, for even after their defeat at Halo 04 the Covenant were still strong, and there were new enemies and variables to contend with that Earth knew nothing about: the Flood, the Forerunner, and their creations, such as 343 Guilty Spark.
After his whirlwind tour of Delta Halo, his introduction to a host of new "friends", and a hitch-hike back to Earth aboard a Forerunner vessel it remains only to be seen how he plans on "finishing this fight".
He has lived most of his life in various military camps on Reach; stolen from his family at age six and his name changed to "John-117", his entire upbringing has been centered around making him a mechanism of military efficiency, utilizing both force and tactics. He displays an odd sort of attitude for a creation of the military: he does not glorify his violent actions, but merely does what he has to do. He does not hate his enemy; he kills them because he knows it is his duty to kill them; to win. "It wasn't [the Chief's] job to make things suffer - he was just here to win battles. Whatever it took." (FoR, p.6)
He is an unconventional Hero, for though he is strong, fast, and tactically brilliant, he is the epitome of none of those attributes, and is surpassed by other Spartans in each area. What John does possess above all is leadership and, to both Cortana's repeated salvation and bewilderment, probability-defying luck.
Cortana is one of thousands of AIs which populate the computer networks of the Human worlds. She is, however, somewhat unique; she is a direct copy of a human being, Dr. Halsey, and lacks behavioral inhibitors of any kind. She is free to do as she pleases, unlike most other military AIs, such as the ONI AI Beowulf, which is strict, sullen, and by-the-book. Cortana, on the other hand, is strong-headed, noisy, and somewhat off-the-wall.
However, her freedom comes at a price. As a "smart" AI, she has an unextendable life-span of approximately 7 years, at which time she will become too smart, suffering from an exponential attenuation of function; she will think so much she will forget to breathe.
At her choosing, she and the Master Chief were specifically paired together for Mjolnir/Spartan II proving trials, in preperation for her placement aboard the Pillar of Autumn for its covert infiltration mission (which was cut short by the Covenant invasion of Reach). In the midst of the Pillar of Autumn's fiery descent to Halo, her safety (as well as that of the UNSC and coordinate data she possessed) could be assured in only one place- the suit of the Master Chief.
However, since her time in Halo 04's Control Center, Cortana has found herself laden with vast amounts of hastily compressed data, the contents of which would surely initiate consequences beyond anyone's wildest expectations. There simply has not been a chance or location to unload and analyze it, something that has frustrated her to no end. The additional burden of carrying this inaccessible information considerably dampened her prowess as well as her ability to concentrate and multitask, as she noticed on several occasions in contact with the Covenant AI aboard the Ascendant Justice. (This information does not appear to be a continuing aspect of the game's story however.)
In spite of this, her spare time has already allowed her to exceed the Covenant's knowledge of their own technology, refining them to create focused streams of plasma energy, a Slipspace system capable of jumps much closer to gravity wells, and enabling her to make copies of herself (further proof of the Covenant's lack of comprehension and ignorant reverence towards their stolen ordnance), allowing her to reek further havoc during fleet engagements.