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.
He is the protagonist and main playable character in Halo: Combat Evolved, Halo 2, Halo 3 and the Reclaimer Saga. 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". 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.
The Master Chief, then known as John, was taken from his home in Elysium City in the Eridanus System, Eridanus II to be exact, at the age of six years old. He was taken by Dr. Catherine Halsey and Lieutenant Jacob Keyes to be trained on the UNSC planet Reach for Spartan training and augmentations. While on Reach, he quickly distinguished himself as the best leader of the SPARTAN II supersoldiers. His real name is John (his last name was deleted from military records, replaced with a numerical tag), but he has been referred to as John-117 or Spartan-117 since his forced induction into the Navy. He continues to lead the few remaining SPARTAN II's, but these soldiers were not present during the events of either game in the series. He is currently awaiting rescue along with the Artificial Intelligence Cortana on the back half of the Forward Unto Dawn in cryonic sleep.
The Master Chief's MJOLNIR battle armor, now up to Mark VI, is designed to be able to hold the most advanced AI's. Cortana is the AI that inhabits the Master Chief's suit, in a crystal network and also an implant that interfaces directly with the Chief's brain. The Chief himself also has a ceramic reinforced skeleton, making his bones virtually unbreakable. This is necessary for him to survive the incredible forces exerted by his armor. He is also more than twice as strong as a regular human the same size (nearly 7 ft. tall) due to chemical enhancements. He is also much faster, and also his reflexes are incredibly fast thanks to a brain surgery specifically designed to cause such effect. These enhancements alone killed 30 of the original SPARTAN II's when they were implemented at the age of 14.
The Chief and the few remaining Spartan II's have all been fighting the Covenant for almost thirty years, since the beginning of the war, when they were still teenagers (all were age 14 when the war started). This was not the original intent of the project (where they were to hold off rebels in the UNSC controlled system), the SPARTAN II's were proved useful in holding off the Covenant, as their augmentations and MJOLNIR powered assault armor suits were the ultimate weapons. They are all about the same age, either 41 or 42 depending on birthdate.
The Chief has long been accustomed to doing things for the greatest good at the sacrifice of individuals, but as of late has realized the importance one person can have.