John 117, commonly known as the Master Chief.
Petty Officer of the Navy John-117 is a SPARTAN-II commando of the UNSC Naval Special Warfare Command. He became one of the most important heroes of the UNSC during the Human-Covenant War. With nearly 30 years of active duty, he is one of the most decorated war veterans in the United Nations Space Command and has earned every known medal in the UNSC except the Prisoner of War Medallion. In 2553, John-117 went missing in action after the Battle of Installation 00, when he and the current Arbiter Thel 'Vadam fired Installation 04B.
As a child, John strove to win at any situation, at all costs. This seems to have been a part of his character even prior to his conscription to the Spartan program, as he pushed himself to win at any game he played, including chess, gravball or King of the Hill. As his career progressed, he transformed his desire to win personal glory to that of victory for his squad, and by extension the soldiers and citizens of the UNSC. John is tenacious to complete any mission at hand, and often disregards his personal safety in order to win. Indeed, he has often emerged victorious from situations many would consider impossible. Still, even a Spartan is not devoid of fear, but John simply acknowledges it and puts it aside, while never openly showing it.
John is generally stoic and taciturn, but not devoid of a sense of humor. He often makes dry remarks about the situation at hand; however, this seems to be more frequent when interacting with Cortana than anyone else, such as the other Spartans.
John is also known as a man of his word and will strive to keep any promise he makes, even at great personal risk. As he was forced to leave Cortana behind on High Charity, John promised to come back for her after dealing with the Prophet of Truth. Cortana chided him not to make a promise when he knew he couldn't keep it. However, despite incredible odds, John did keep his promise, as he fought his way through the Flood into High Charity to retrieve her. Upon finding her, Cortana was in a weak and damaged state. Nearing rampancy, the Master Chief told her, "You know me. When I make a promise..." to which Cortana replied "You keep it." This encouragement gave her the strength to compose herself after enduring the mental tortures of the Gravemind. John holds great personal respect for his trainer and mentor Chief Petty Officer Mendez, as well as Dr. Halsey, who he considers a mother-like figure. He appears to deny Halsey's claims about his extraordinary luck at several points in the Halo Legends episode "The Package".
In a strange twist of fate, one of John's closests friendships was with the construct Cortana, an artificial intelligence "born" from the mind of Dr. Halsey herself. Cortana was initially assigned to him for his mission to capture the Covenant Prophets. Despite some initial hesitation towards working with an AI out of fear of conflicting directives, John and Cortana quickly formed a formidable team and John realized quickly that Cortana would be a great help instead of a liability. The two's relationship evolved from a partnership to a close friendship over the course of Halo: Combat Evolved, due both the duration of the mission and the life-or-death situations they struggled through. During their adventures, John showed absolute trust in Cortana, believing in her even when Lord Hood was unsure of her reliability, being willing to trust her enough to risk Earth on her word that she had a way to stop the Flood. The two also had similar personalities: they both had a propensity to blow things up, with John commenting he wasn't sure which one of them was better at it. The pair also tended to enjoy executing high risk plans. After Cortana explained to John his strategy to use a disabled bomb to blow up a Covenant Carrier was crazy, he offered to leave her behind. Instead, she quipped, "Unfortunately for us both, I like crazy."
Although John has difficulty understanding the 'undisciplined' lifestyle of civilians, he is fiercely protective of their lives and humanity as a whole. The mass slaughter of civilians at the hands of the Covenant was enough to drive even the stoic John into a merciless rage. After the massacre of Draco III, John and his Spartans remained on-site until every Covenant soldier responsible for the atrocity was dead.
John is known to show an exceptional care for soldiers under his command, while at times viewing them as a liability rather than an asset. By 2552, after his experiences on Halo, John acknowledged to Corporal Locklear that, armor and enhancements aside, there was little difference between a 'normal' soldier and himself. This is not to say that John was previously without compassion. During the Battle of Installation 04, when searching for the survivors of the Pillar of Autumn, a Marine named Fitzgerald was wounded by a Grunt with a Needler. John-117 retrieved a first aid kit and expertly treated the wound, taking time to ask the Marine if he was alright. By 2552, most members of the UNSC held "the Master Chief" in complete awe, even amongst his Spartan comrades. His exploits and prowess quickly made him a legend in his own time, and his mere presence on a battlefield was enough to inspire the hope of victory regardless of the odds.