Master Chief Petty Officer of the Navy, John-117, is a SPARTAN-II Commando of the UNSC Naval Special Warfare Command. He served as one of the most important figures of the Human-Covenant War and the Alpha 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 slowed the Flood threat.

He is referred to universally by humans simply as the Master Chief, Chief for short and Sierra-117. The Covenant is aware of the power of the Spartans, and refer to the Master Chief as the Demon. Meanwhile, John is referred to as Reclaimer by both 343 Guilty Spark and 2401 Penitent Tangent, although this seems to be a universal title given to all humans, rather than specifically referring to John-117.