Master Chief Petty Officer John-117, is a SPARTAN-II of the UNSC Naval Special Warfare Group. With almost 30 years of active duty, he is one of the most decorated soldiers in the UNSC Navy. In 2553, John-117 went missing in action after the Battle of Installation 04(II) when he and the Arbiter eliminated the Flood threat [1]. He is the main protagonist in the Halo Series. He is also known as the "Demon" by the Covenant and the Elites, "Master Chief", "Spartan", "Spartan-117" "Sierra 117", "Chief" by the UNSC and "Reclaimer" by 343 Guilty Spark.

Master Chief was only six years of age when The UNSC government took him and 70 other children for life long training. In his place they left a flash clone to be a placeholder for his parents without them knowing. His first name was John but, was assigned number 117 for the duration of his training. He was determined to be as near to a perfect human being as it is possible to be and so fit to undertake the physical and mental training necessary to become a Spartan II super soldier. Like the other candidates John was strong and swift and brave, but innate luck was what set him out from the other Spartans. This quality was noted by Dr Catherine Hallesy at the outset of the selection process and it was because of it that John was chosen to test the new Mjolnir armor that the Spartans were to be issued with.

The Spartans were trained under CPO Mendez for a total of eight years. In the first two years they undertook physical ,intellectual and weapons training. When the Spartans where eight years old, they were sent on a training exercise. The Spartans dropped from a pelican dropship over an area of wilderness. Each had one piece of the map leading to their extraction zone. The twist to the exercise was that the last Spartan to make it onto the extraction Pelican would be left behind. John quickly realized that teamwork was the key to the exercise and rallied the other Spartans worked together to complete their mission. This showed that he had learn from an earlier failure where he had abandoned his team in order to complete the objective. When the Spartans arrived at the vehicle they discovered that it was guarded by men under orders to prevent the Spartans from escaping. Once again John showed his leadership potential and organized the young Spartans to take down the guards and escape. He then ensured that all the others got on the Pelican before him, meaning that he would be left behind rather than any of them. However Hallesy was so impressed by his leadership qualities and willingness to sacrifice all for his team mates that he was not only allowed to come with them but was promoted to squad leader. At the age of 14, Master Chief underwent both physical and cybernetic augmentation (including Carbide ceramic ossification, Muscular enhancement injections, Catalytic thyroid implant, Occipital Capillary reversal, Superconducting fibrification of neural dendrites, and Neural implants). Only 32 of the 70 children in the Spartan program survived the augmentation process