Master Chief

Master Chief Petty Officer John-117 is a Spartan-II commando of the UNSC Naval Special Warfare Command who became one of the most important UNSC heroes during the Human-Covenant war. With over thirty years of active duty, he has become one of the most decorated war veterans in the United Nations Space Command, earning every known UNSC medal except the Prisoner of War Medallion. His goal is to save humanity from hostile alien races.

John-117 is the protagonist and main playable character in both the Halo trilogy and Reclaimer Saga. John is only ever referred to by name once in the Halo trilogy of games: by Cortana in the ending cutscene of Halo 3 prior to the credits. In Halo 4 -the beginning of the Reclaimer Saga, both Catherine Halsey and Cortana refer to him by name twice. Humans generally refer to him as the "Master Chief", "Chief" for short, or "Sierra-117". The Covenant, aware of the lethality of the Spartans, refer to John exclusively as "Demon". However, after the Great Schism and the Sangheili alliance with the UNSC, both Thel 'Vadam and Rtas 'Vadum alternatively referred to him as "Spartan". Forerunner enemies and their AI constructs generally refer to John as "Reclaimer", although they seem to use this term in reference to all humans, and not John specifically. Interestingly, the Ur-Didact in his apparent disdain for humanity, chose to curtly address John as "human". However, after John had successfully infiltrated his way into Mantle's Approach, the Ur-Didact chose to refer to him more respectfully as "warrior" before their final confrontation atop the light bridge Master Chief Battles in Chicago against the covenant on the Cover of Halo 2, He recently visits the city in the whole franchise including the Trailers