John-117

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. Halo 3, Epilogue

He is the main protagonist and main playable character in Halo: Combat Evolved, Halo 2, Halo 3 and the upcoming new trilogy. In the games, John is only ever referred to by name once, by Cortana, in the ending cutscene of Halo 3 before the credits. 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", although, after the Great Schism, Thel 'Vadam and Rtas 'Vadum called him Spartan. 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.