Master Chief, military rank:
A Master Chief Petty Officer (abbreviated MCPO) is the most senior enlisted person (E-9) in either the Navy, Coast Guard, or any similar military branch. The rank is comparable to a Sergeant Major or a Chief Master Sergeant in the other branches. The correct mode of address for a Master Chief is "Master Chief" or "Chief."
A Master Chief's shoulder insignia in the United States Navy is composed of three yellow downward facing chevrons topped by a downward-facing arch, and on top of that, an white eagle with two small white stars above either wing. In the space between the chevrons and the arch is a small white sun-like symbol. These symbols are mounted on a navy background.
The Master Chief, Halo a.k.a. John-117
In the FPS Halo, the Master Chief is, to steal blatantly for a write-up I saw wandering through here today, the kind of guy who would lead us into the valley of the shadow of death, and we would fear no evil because he is the baddest mother in the valley. And I'm serious about that. Yes, I know he's just a character in a hack-and-slash game, but I can't help but get a little attached to the guy.
Dr. Halsey, an agent of a shadowy milintel operation called Section 2, part of the Office of Naval Intelligence, kidnapped the Master Chief at the age of six from his home planet of Eridanus 2. He was then enlisted in a secret project codenamed SPARTAN-II along with seventy-five other children (they were replaced on their homeworlds with "flash" clones, who later died of congenital diseases due to errors in the cloning process). Years of intensive military and academic training followed during which the Master Chief was transformed into the battle-ready beast you see in Halo. The Master Chief's strength is not in speed or intelligence or even combat effectiveness, as is evidenced in Halo: The Fall of Reach, but some underlying nobility that gives him his extraordinary ability to lead. As well as luck. Lots of luck.
After the "boot camp" period, the Spartans were subjected to a series of risky biological enhancements, including carbide ceramic ossification of their bones (which made them nearly unbreakable), muscular enhancement injections (to increase muscular density and decrease lactic recovery time), catalytic thyroid implants (to boost growth of skeletal and muscular systems), and superconducting fibrification of neural dendrites (to reduce reaction time). Thirty-three Spartans survived the augmentation unharmed, a dozen were permanently injured beyond combat effectiveness, and the rest died during the process.
The surviving Spartans were used as elite strike teams for periodic revolts on different planets, but John (or the Master Chief) felt that this was somewhat less than an ultimate goal for the Spartan "super-soldiers." And, ironically, he was right. In 2525, the colony Harvest was completely destroyed. A battle group sent to investigate the loss of communications was immediately attacked by what was later discovered to be humankind's first contact with the Covenant, an alien race bent on destroying humankind. Long story short, the Covenant made mincemeat out of the human fleet for years, and in return the Spartans make mincemeat of the Covenant- but only superificial ground victories. Earth itself is in danger.
Of course, cue heroic yet suicidal mission. The Spartans are tasked with capturing a Covenant vessel, driving it to Covenant space, capturing one of their leaders (a 'Prophet'), and hammering out a truce. Easy as pie, considering no Covenant ship has ever been captured and they are technologically superior to humans in almost every way. An old destroyer, the Pillar of Autumn, is fitted out to go after the Covenant. Just before they leave, however, the planet the Spartans have been training on, Reach, is attacked by a huge Covenant armada. The majority of the Spartans are sent planetside to fight the Covenant on the ground, but three Spartans, including the Master Chief, volunteer for a mission in space which involves destroying a spy craft that did not erase data relating to the position of Earth. They succeed, but the other one Spartan dies, the other is seriously injured and put in stasis, and the Master Chief returns with the injured Spartan to the Pillar of Autumn. The Pillar jumps out of the system, directed towards an ring-shaped structure in space (Halo) by Cortana, the ship AI, who plays a very important part in later events.
This is the point at which the game begins, but the information I have covered is contained the the novel based on the game called The Fall of Reach. I thought it was going to be unbearably bad (most game-based books are), but it's surprisingly thoughtful and well-written. The author actually has a degree in chemical physics, and most of the scientific details are accurate. I recommend it if you're into the game.
If you're really into the game, however, Nightmare Armor has begun creating full sets of Mjolnir armor (minus the extras, of course) which will run you about $3,500. A more affordable option is the Master Chief action figure, made by Joyride Studios, at about $15.00 a figure. They have all of the armor colors (including active camouflage and "battle damaged") as well as the vehicles, two models of Marines, Cortana, and the ever-present Sergeant Johnson.