Master Chief Petty Officer of the UNSC Navy, John-117 is a SPARTAN-II Commando of the UNSC Naval Special Warfare Command. With nearly 30 years of active duty, he is one of the most decorated soldiers 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 eliminated the Flood threat.

He is more commonly known as "Spartan" by Thel 'Vadam and Rtas 'Vadum, "Demon" by the Covenant and other Sangheili until the Covenant Civil War, after which the other Sangheilis also started to refer to him as "Spartan", "Master Chief" and "Chief" by Humans, and "Reclaimer" by both 343 Guilty Spark and 2401 Penitent Tangent.

Born in about 2511, John lived with his mother and father in Elysium City, on the colony world Eridanus II. It is and has been said that John has brown hair, freckles, and a small gap between his front teeth. In Halo: The Flood, his mother was mentioned as kind and smelled of soap. At the age of six, he was identified by Dr. Catherine Elizabeth Halsey as one of 150 preliminary candidates for the SPARTAN-II Program. John was an ideal physical candidate, standing a head taller than his schoolmates, having perfect physical proportions, possessing great strength, exhibiting quick reflexes, and having an aggressive drive for winning. When Dr. Halsey and Lieutenant Jacob Keyes visited Eridanus II in 2517 to study him, they were impressed with his intellect and his luck. In a final test of his viability as a candidate, he was asked to determine which side an old coin would land on. He watched the coin as it flew in the air and caught it before it could land, correctly stating which side was face-up. Later that same year John, and 74 other six-year-olds were "conscripted", covertly kidnapped from their homes and replaced with Flash Clones, all of which would later die of natural causes to cover up the kidnappings. John along with the other candidates are taken to Reach, it is here that Dr Halsey informs them of the SPARTAN-II program. The next morning John discovers his service tag, stitched into his training uniform, and from now on he is only known as John-117.

John began a new life, training with other Spartans under Chief Petty Officer Franklin Mendez for eight years he learns history, military strategy, weapons, and physical fitness. He bonded early with Kelly and Sam, who would be the only close friends he would ever have.

When the Spartans were eight years old, they were sent on a training mission in which they were dropped over a forest located deep in one of Reach's snow-capped mountain ranges and expected to get safely to their extraction Pelican. The children also had instructions to leave the last child arriving behind, or the punishment would be painful, but John-117 knew that he could not do that. After telling the Spartans where to meet up, the Spartans looked to him as their leader a role he was initially surprised to be given but took on. When the group came upon the Pelican, it was guarded by armed men. Those men were actually UNSC soldiers, but were not in uniform, and in addition to the fact that the Spartans may have never seen Marines before, therefore mistaken for a hostile threat. John showed early acumen in critical thinking and tactics, not wishing to take it for granted that guards would be friendly toward them. He came up with a plan to take the men down, capture the dropship, and ensure the safe extraction of every team member, as he made sure he was the last person aboard. To do this he ended up hijacking the dropship with the help of Déjà, and beat down the men with stones and many feet, causing severe injuries among the Marines. Chief Mendez was not pleased, as one, John attacked his Marines, and two, no one had been left behind. But Dr Halsey recognized his initiative, and promoted him to Squad Leader. After this despite the other Spartans rising in rank and at least a couple rising to lead their own teams, he remained the de facto leader of the Spartans.

At the age of 14, John underwent the dangerous and painful process of SPARTAN-II Augmentation Procedures that caused most of the Spartan candidates to die, or be physically disabled. John was one of 33 Spartans who made it through the process unscathed, while the rest who could still operate were sent for jobs in the Office of Naval Intelligence. At only 14 years old, it is said that John had a body of an 18 year old Olympic athlete, the augmentation process gave the Spartans faster reflexes, greater strength, better eyesight and made their bones near unbreakable.