Until the age of six, John lived a normal life with his family on the colony of Eridanus II. His conscription into the SPARTAN-II program was to shape much of his personality in the years to come. However, despite his difficulty relating to non-SPARTAN personnel, John nevertheless showed great depth of character throughout his many years of service.
As with all Spartans, John strives to win at any situation, at all costs. Although the drive to win was ingrained to the Spartans as part of their indoctrination, John appears to have possessed such an inclination even prior to his conscription; as a child, he would win at any game he played, including chess,
gravball or King of the Hill. He is extremely determined to complete any mission at hand, and often disregards his personal safety to do so. He is also noted for his exceptional luck, which, along with his combat skills and unyielding persistence, has allowed him to emerge victorious from situations many would consider impossible. Because of his many years of constant combat and military conditioning, John is not as emotional or sensitive as he once was, especially to fear. Although he is not devoid of fear, he simply acknowledges the feeling and puts it aside. He is calm in the face of danger but not foolhardy, as demonstrated during his first encounter with the Gravemind, when he warned the Arbiter that his arrogance was likely to get him killed.
John is generally stoic and taciturn, but not devoid of a sense of humor. He often makes dry remarks about the situation in hand; however, this seems to be more frequent when interacting with Cortana than anyone else, such as the other Spartans. He does not see himself as a superior being and fears for the lives of others before his own, putting himself in harm's way for the safety of civilians, and fellow soldiers, and does not judge others. Indeed, he is known to show an exceptional care for soldiers under his command and has shown great respect for the baseline humans he has fought alongside. An example of this is during the Battle of Installation 04. When making his way up to the
Silent Cartographer, two Helljumpers, PFC
Hosky and
another Corporal, were killed by two Hunters. Afterward he regretted their loss, blaming himself for their deaths. This, and his reaction to
Johnson's death on
Installation 04B show that he does exhibit care and sympathy for soldiers under his command, even if he does do a good job of concealing his feelings. He was also impressed by the humans who comprised his team following his escape from Installation 04, including
Sheila Polaski and
Corporal Locklear, regarding the latter "as much a warrior as any Spartan"
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. Unfazed, 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's unfaltering, determined nature also serves to mask his feelings on many occasions, as he prefers to devote all attention to the goal at hand. He is cautious and observant, only making changes in his plans if they are of the utmost necessity. John realizes his duty as a Spartan is to serve and help those in need, and can work professionally with anyone he is sent into battle with. 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 cold rage. After the
massacre on Draco III, John and his Spartans remained on-site until every Covenant soldier responsible for the atrocity was dead.
ohn greatly dislikes being kept in the dark, ambushes and fighting human soldiers. Once, when cornered by four large ODSTs in a gym, he did his best to avoid a confrontation. In the end, he was forced to defend himself with lethal force. While killing Covenant never placed a burden of guilt on John, ending the lives of fellow human beings was something he never felt comfortable remembering. As a soldier, John knows when it is appropriate and inappropriate to fight, and has great control over his emotions thanks to a clear state of mind. Due to his augmentations, he reacts much faster than an average human, seeing events at a fraction of normal speed. It is almost a clairvoyant state of mind, and as a result of this, has an acute intuitive insight and perception of any situation combat or non-combat. He has been known to have vivid and long-enduring memories of the past, such as in dreams. No matter how well-adjusted he is, however, John always seems to need to have a quiet place away from crowds after a battle in order to feel at peace.
While reflecting on his life during his battle with the Didact John stated that he did not remember anything from his childhood from before his conscription: not his name, his family, his home, except the King of the Hill game he played with the other children in his school's playground. Through thirty years of warfare he was always prepared to die in battle, but when he was about to die by Didact's hand on Gamma Halo, he discovered he was not ready to die. After defeating the Didact, John continued to obsessively embark on new battles even after receiving an R&R order from Lord Hood.
When Lord Hood offered John a commission, even suggesting he could be conferred the rank of admiral, the Master Chief jokingly replied, "The Admiral doesn't have quite the same ring to it"
