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.
http://halostory.bungie.org/masterchief.html
For two decades, Fisher has been involved in numerous armed conflicts throughout the world, many of them unconventional. Over the course of his extensive service career, he has served in hot spots such as the Persian Gulf (i.e. Iraq, Kuwait, etc.), Colombia, Senegal, and Afghanistan as well as taking special assignments in North Korea, Russia, Panama, the former East Germany, and the Republic of Georgia. He has not only survived, but has also excelled in the field of covert operations through hard work, insatiable curiosity and brutal honesty. Combat, espionage and constant training have defined his adult life; his tactical experience has become part of his instinct. He is quiet, instinctive, and observant: somebody who watches from the outside. Even outside of work he is most comfortable on the fringes of society, keenly observant but still removed.
Though fully aware and confident of his abilities, Fisher understands that his survival has often been a gift of chance. He knows he is human and fallible, and he does not want to die. Highly-trained, skilled and experienced, Fisher possesses a distinctly dark sense of humor, which he uses to control his fears and keep himself calm while in the field. Personality-wise, Fisher is abrasive and honest, and has little patience for niceties and even less for lies, especially when the immediate problem presents itself. Unlike younger operatives, he is not a blind idealist.
But in order to achieve the discipline required for his work, Fisher has had to bury certain aspects of himself. His ability to detach himself emotionally from the immediate situation in order to better achieve his objectives is an exceptional attribute. Though, on rare occasions, his emotions have been known to surface. A man of few words, he is truly himself only when he is on the job – he has, in a sense, become what he does. He has acquired an admirable collection of scars, war experiences, and a place in military history. He has little left to prove to the world and he has no interest in glory. If he fights, it is because he believes he is capable of provoking positive change.
http://splintercell.wikia.com/wiki/Sam_Fisher
Poll