Also known as "Master Chief or just Chief"
The Master Chief was the sole Spartan to leave the Reach system aboard the Pillar of Autumn (with the exception of Linda-058 in cryo). For all he knew, he was the last Spartan alive. All of humanity depended on him, for even after their defeat at Halo 04 the Covenant were still strong, and there were new enemies and variables to contend with that Earth knew nothing about: the Flood, the Forerunner, and their creations, such as 343 Guilty Spark.
After his whirlwind tour of Delta Halo, his introduction to a host of new "friends", and a hitch-hike back to Earth aboard a Forerunner vessel it remains only to be seen how he plans on "finishing this fight".
He has lived most of his life in various military camps on Reach; stolen from his family at age six and his name changed to "John-117", his entire upbringing has been centered around making him a mechanism of military efficiency, utilizing both force and tactics. He displays an odd sort of attitude for a creation of the military: he does not glorify his violent actions, but merely does what he has to do. He does not hate his enemy; he kills them because he knows it is his duty to kill them; to win. "It wasn't the Chief's job to make things suffer - he was just here to win battles. Whatever it took." (FoR, p.6)
He is an unconventional Hero, for though he is strong, fast, and tactically brilliant, he is the epitome of none of those attributes, and is surpassed by other Spartans in each area. What John does possess above all is leadership and, to both Cortana's repeated salvation and bewilderment, probability-defying luck.
The story follows...
The modern Spartan program (in contrast to its ancient Greek counterpart ) was initiated in 2491, an element of the ORION project. Though not much is known about the first generation of warriors (their training, augmentation, goals, etc.), they were successful enough to garner the necessary consent for a second phase, aptly named SPARTAN II. Under Dr. Halsey, the Spartans would be developed in conjunction with the MJOLNIR armour project, the culmination of a powered exoskeleton technology that had been used for some time by the UNSC, and when discarded, by frustrated cargo handlers everywhere. The initial goal of the SPARTAN/MJOLNIR project was to provide a surgical strike team capable of dealing with Human rebellions quickly and efficiently, averting the use of large scale military force, and preventing the civil war, and tremendous loss of life, that would have inevitably ensued.
A total of 75 children, approximately age six and selected for their specific genetic dispositions (that is, being as close to perfect as science could determine), were "recruited". They were replaced with congenitally defective flash-clones to cover the enlistment, and trained, primarily on and below the surface of Reach, by one Chief Petty Officer Mendez, arguably the best Drill Instructor in the Navy. Their family names, whatever they may have been, were stripped, purged from record, and forever replaced with a 3 digit designation. Their upbringing would be defined not only by gruelling physical routine, but by a comprehensive academic tutelage under Deja, a Class II AI created specifically for the SPARTAN II project.
In March of 2525, on the orbital ONI Medical Facility above Reach, the then 14 year old candidates underwent a series of drastic and inadvisable operations to dramatically enhance their skeletal, muscular, optical, and nervous systems (FoR p.56). Of the original 75, a reported 33 survived the processes; 30 were lost, and 12 irrevocably crippled (though still used by ONI in research and ops planning). (Note: The exact number of Spartans to survive the conflicts to follow varies dramatically from source to source. Check here for a closer examination.)
Both before and after their final union with the MJOLNIR armour, the SPARTAN II's, with their overwhelming physical abilities, improvisational skills, and keenness for adversity, handled their operations with a profound efficiency: as of August 27, 2552 they had suffered only 3 KIA's and 1 too wounded to continue active duty, giving them the best record of any UNSC unit. Of course, they were officially listed as only MIA, part of ONI Section 2's successful propaganda campaign to boost morale; a program spearheaded by the SPARTAN II's and their glorious triumphs in battle, though strangely silent concerning Humanity's slow and imminent loss of the war against the Covenant.
Since their harrowing escape from Reach and the destruction of nearly 500 Covenant vessels consumed in the explosion of the 30km long Covenant Command Station, Unyielding Hierophant, the arduous task of defending Humanity from the ever gargantuan Covenant fleet (wise to the Earth's location) fell into their "big, green, armoured hands." Though their numbers were bolstered to 7 (or 8, including the abducted Kelly) before their return to Earth, the story's continuation focuses only on Spartan-117, with no mention of his comrades. Let's hope one is enough.
His armor is as follows...
In Bungie's franchise, Halo, the MJOLNIR battle armor is worn by the SPARTAN-II soldiers, inclusive of the protagonist known as 'Master Chief' (also known as John-117 in the Halo novels). The sole origin and first public encounter with the MJOLNIR battle armor, however, took place in a previous video game series by Bungie, titled Marathon. The suit's name, 'Mjolnir' (myohl-nir, IPA:Mjolnər), is derivative of Norse Mythology. Mjolnir was the hammer of the thunder god, Thor.
The versions of the MJOLNIR Armor in the Halo universe is completely separate from the Marathon series. There are currently six iterations of the MJOLNIR armor depicted in the Halo games and the novels. There has been many other verions of this armor that bungie never released.
The MJOLNIR Mark IV was chronologically introduced in Halo: The Fall of Reach, and is a radical redesign of the suit taking advantage of many technological leaps as the Mark I-Mark III armors required massive power consumption by being plugged to a generator, which was rendered impractical.
The MJOLNIR Mark IV armor's HUD contains an integrated computer system which continuously monitors weapons, ammunition, biological function, and motion (through an intelligent motion-sensing radar), and is capable of linking with weapons to generate an accurate targeting reticule. The SPARTAN-IIs were the only people that could wear the armor due to their augmentations (namely their hard bones and enhanced reflexes), as a standard Marine had his bones crushed when testing the armor in Fall of Reach.
The Mark IV Assault armor was fielded from November 27th, 2525 to August 29th, 2552.
Prior to the Fall of Reach, the Mark IV suits were upgraded to Mark V, which allowed SPARTANs to house A.I. inside their helmets, as well as carry energy shields as a result of reverse-engineering the Jackal's shields. The Master Chief demonstrates the full lethality of the suit towards the end of the book, and was currently used in Halo: Combat Evolved.
Upon returning to Earth, the Master Chief's heavily damaged Mark V suit was replaced with a Mark VI, which gave a simplified H.U.D., a faster recharging energy shield, made falling damage almost non-existent, as well as removing the need to use standard medical kits.
Marathon - The MJOLNIR Mark IV Cyborg
During the course of the Marathon games, various text terminals mention of ten MJOLNIR Mark IV cyborgs, which represent "all of the destructive knowledge of mankind." The identity of the protagonist is never solidly stated in the games. The manual states that he is a security officer aboard the UESC Marathon. However, in the Marathon 2 preview, the AI Durandal reveals the following:
"Were you ever curious about the tenth cyborg which was supposed to be on the Marathon? Why could Leela find only nine? Was it ever activated? Was it destroyed during the defense of the colony? You are right to be curious."
This and other clues have led many fans of the series to the conclusion that the player is the tenth MJOLNIR Mark IV cyborg, though as with all Bungie games, speculation is common.
Real-life MJOLNIR Replicas
Many replicas of the MJOLNIR armor can be identified, such as the work of Nightmare Armor Studios, 'Chris Bryan', and of the website 'Briar's Armor'.
Spartan 117 is a superhuman android monkey fluff.