Master Chief

Although his official military designation is Spartan-117, he is most often referred to simply as the "Master Chief" or "Chief", short for his rate of Master Chief Petty Officer. (In the Navy, enlisted "ranks" are referred to as "rates". The use of the word "rank" for enlisted Navy personnel is incorrect.) The assignment of Navy enlisted rates to him and other SPARTAN-IIs resembles the usage of these rates by the United States Navy, a correlation further evidenced by the fact that his unit is attached to the UNSC's "Naval Special Warfare" division. In this usage, personnel holding rates that would traditionally be associated with purely administrative tasks become equivalent to Army or Marine Non-commissioned officers that have been assigned to lead a Special forces unit, thus limiting their responsibilities to the command of their squad.

The character of the Master Chief is a SPARTAN-II Super Soldier, a biologically-enhanced and cybernetically-augmented commando originally assigned to the task of putting down rebels and insurgents within the United Nations Space Command. When the powerful alien empire known as the Covenant attacked, the Master Chief and the other Spartans had a priority shift. Extensive surgical alteration coupled with a ruthless lifelong training regimen has resulted in Master Chief being significantly stronger than even the strongest natural human, and his strength and fighting ability are further augmented by the advanced MJOLNIR power armor he wears.

According to Bungie Studios, The Master Chief stands 7 feet tall and weighs approximately 'half a ton' in full battle armor (2.1 meters and 454 kilograms). He is approximately forty-one years old at the time of the events of Halo: Combat Evolved (2001). Between Halo: Combat Evolved (2001) and Halo 2 (2004), the Chief's armor is upgraded from MJOLNIR Mark V to Mark VI

Sergeant Major A.J. Johnson

Is a Marine who leads a group of human marines against Covenant and Flood assaults throughout the first and second games. He is best known for his tough-guy attitude and outrageous pep talks. Yet, for all his talk, Johnson is the spitting image of what a UNSC Sergeant should be. He also has managed to live through many battles with the Covenant and an infection from the Flood. (In-game in Halo, Johnson can die in individual battles, but survives in canon. In Halo 2, whenever Johnson makes an appearance in normal gameplay, he is invincible). Johnson's encounter with The Flood is seen by the Master Chief via recorded video. The encounter is chronicled in the Breaking Quarantine story of the Halo Graphic Novel, illustrated by Tsutomu Nihei, which involves Johnson coming face to face with the Flood and single-handedly defeating them as he desperately tries to escape. The story begins with Johnson having to shoot one of his own infected men. There he sees first-hand what the Flood are, and he begins to flee. The Flood quickly grow in numbers and as Johnson crosses a light bridge the Flood manipulate the controls to turn it off, however Johnson's athleticism comes through as he is able to jump across the remaining portion of the bridge, and tosses a grenade to keep them at bay as he succeeds at his escape.

In the beginning of Halo 2, a Master Gunnery Sergeant on the orbital station Cairo asks him how he survived and made it back to Earth after the destruction of the first Halo. He answers that the information is classified.

In all actuality, according to the novel Halo: First Strike, Sergeant Johnson survived the explosion of the first Halo because he escaped the explosion in a Pelican drop ship with 3 others. The Pelican hid in the debris of Halo before being found by Master Chief and Cortana whilst they were still flying in a Longsword fighter. As for how he survived his encounter with the Flood on Alpha Halo, First Strike details how the Sergeant has a rare medical condition called Boren's Syndrome, which results from a heavy exposure of the nervous system to the electromagnetic fields emitted by plasma. Sergeant Johnson obtained this condition when he used a crate full of plasma grenades in a past conflict against the Covenant during the Siege of Paris IV to defend his squad until they could be evacuated. This condition is what saved him from the Flood's ability to consume their hosts' minds and modify their bodies, as the abnormal frequencies in his nervous system served as a buffer to keep the Flood out of his mind. Johnson could have died from ONI's experimentations had the Master Chief given the complete combat data on the Flood, including Johnson's survival in the report. The Master Chief gave the regular combat data and omitted the reasoning behind Johnson's survival. This was later supported in Halo 2 when Master Guns asked Johnson how he survived. Johnson gave a simple "Classified information" since the Master Chief at one point swore Johnson to keep it a secret.

In Halo 2, he was awarded the Colonial Cross, led UNSC forces to drive the Covenant from New Mombasa and Delta Halo and was captured along with Miranda Keyes by the Covenant but managed to escape when the Arbiter arrived and attacked his captors. He took advantage of the distraction, allowing him to commandeer a nearby Scarab and request the Arbiter's help in defeating Tartarus.

Cortana

Cortana is a "smart" artificial intelligence that was constructed from the flash-cloned brain of Dr. Catherine Elizabeth Halsey, the creator of the SPARTAN-II Project. The Doctor's tissue was cloned, and then the synaptic networks became the base of Cortana's processors. The term "smart" is a misnomer, as all artificial intelligences are exceptionally brilliant, compared to human standards. It simply means that her creative matrix is allowed to continually expand, unlike "dumb" A.I.s, which have limited matrices. This grants her the ability to learn and adapt beyond her basic parameters, but at the cost of a limited "lifespan" of only seven years. Her primary mission was to assist the Spartans in the capture of the Covenant High Prophets, and then to negotiate either a truce or peace treaty.

Cortana has no physical form, unlike 343 Guilty Spark's avatar. However, she maintains the ability to communicate with a strong female voice through available communications systems, and, if the correct equipment is present, to project an holographic avatar of herself to ease interaction with humans. She projects the image of a woman rendered in purple light, said to be a younger version of Dr. Halsey. Later her appearance changes to a similar blue figure with longer hair.

Originally, the core of Cortana was contained in a disk, which could be inserted into most systems; however, the MJOLNIR Mark VI armor system is apparently equipped to allow her to transfer from system to system with a mere touch of the palm. Cortana was designed to infiltrate any security system, and once she is physically inside, there is little that can stop her from taking absolute control.

Aside from that, she is very proficient in the coordination and use of UNSC and later Covenant military hardware. Because one of the UNSC's primary uses of artificial intelligence is to oversee the targeting of point-defense and ship-to-ship weaponry, space navigation, and calculating Slipspace jumps, Cortana can operate ships and space stations by herself. But, for Cortana at least, this is only a secondary function. Cortana was able to command the cruiser U.N.S.C. Pillar of Autumn to great effect over the planet of Reach and the astronomical phenomenon known as Halo against hostile Covenant space forces.

The Arbiter

The rank of Arbiter is bestowed upon a Sangheili (known to many as Elites) by the High Prophets during a time of extraordinary crisis. The Arbiter acts as a sort of field commando, going on missions specifically dictated by the Prophets to resolve whatever difficulties the Covenant is experiencing. During incidents such as the Taming of the Hunters, the Grunt Rebellion, and most recently, the threat of heresy, and but not including, the Covenant Civil War, the Arbiter of the time has emerged to lead the Covenant to victory. Many, especially Grunts, see the Arbiter as their Savior or protector. It should be noted that his role is considered "The Will of the Prophets" or similarly "The Blade of the Prophets". As the normal strike forces of the Covenant are the Arm, the frontline Arbiters are seen as the manifestation of the Hierarchs' desire. Every Arbiter was "created and consumed in times of extraordinary crisis", and each has been martyred in the undertaking of his momentous tasks. The corpses (or perhaps only memorials of some sort) are housed in identical caskets stacked upon each other in the great Mausoleum of the Arbiter, located on High Charity. In the center of this room a floating pod contains the sacred armor of the Arbiter, highly decorative and fully functional (although some subsystems are out-dated, such as the active camouflage) despite its apparent age. The mausoleum houses 168 visible caskets (and more may be set underground), reaching high into the air, and references of Arbiters settling the Grunt Rebellion and aiding the Taming of the Hunters would indicate that the office has existed for quite some time - before the Covenant as we know it was fully formed and its castes defined. The Arbiter can only be an Elite (the characteristic armor is tailored to the Elite anatomy), as they are traditionally seen as the guardians and protectors of the Covenant.

As a member of the Covenant, it is believed that the Arbiter is held in the capacity of maintaining the integrity of the Covenant, and his creation is necessitated only in momentous events like those mentioned above. Of course, he also serves as a political tool, an embodiment of the Hierarchs' power as well as an example of true faith, a warrior-martyr showing loyalty at times of division.