Master Chief (Mark VI GEN2)

Master Chief Petty Officer John-117, also known simply by his rank as the "Master Chief" or (as is common in military culture) simply as the "Chief", is a Spartan-II supersoldier of the UNSC Navy's Special Weapons division. He served as one of the most important figures of the Human-Covenant War. After over thirty years of active duty, he is one of the most decorated war veterans of the United Nations Space Command.

John has demonstrated easy familiarity with various weapons, though unless a mission specifically requires something, has shown a preference for the MA5 series of assault rifles as his mainstay.

Cortana is a "smart" AI formerly in service with the United Nations Space Command. Unique and exceptionally powerful even among smart AIs, she was one of the most important figures in the Human-Covenant War, and served as John-117's partner.

Cortana is currently slowly undergoing rampancy. Rampancy is a terminal state of being for artificial intelligence constructs in which the AI behaves contrary to its programming-imposed constraints. Traditionally, this is linked with the AI developing a longing for godlike power and contempt for its mentally inferior makers. When rampancy occurs, there is no way to restore the AI to its previous state and the only alternative is to destroy it before it harms itself and others around it. Rampancy can manifest itself in a number of ways in virtually any self-aware technological intelligence. In the case of human AIs, the condition is exclusive to the "smart" variety, occurring after the AI has been in operation for seven years and has gathered too much data in that time period.

UNSC Infinity (INF-101) is an experimental Infinity-class supercarrier of the UNSC Navy commissioned following the end of the Human-Covenant War. Built in secret utilizing technology recovered from Forerunner and Covenant sources during the war, Infinity is the UNSC's largest and most advanced multi-role vessel to date. Infinity serves under the direction of Fleet Command and was originally captained by Andrew Del Rio, and later Thomas Lasky.

The United Nations Space Command (UNSC) is the military, exploratory, and scientific agency of the Unified Earth Government (UEG) and has acted as the emergency governing body of the human race.

The Covenant is a massive religious hegemony of multiple alien species that completely controlled a large portion of the Orion Arm in the Milky Way Galaxy for thousands of years. The Type-25 plasma rifle, used by Convenant infantry, is a battery-powered directed-energy weapon that is capable of fully automatic fire. The Type-25 Directed Energy Pistol, the plasma pistol, is a Covenant infantry weapon

The Sangheili, or "Elites" as they are known by humans, served as the military leaders of the Covenant. The Sangheili are the only race within the Covenant permitted to build and command starships within the Covenant, though the crews are always composed of a mix of races. The Jiralhanae, or "Brutes" as they are known by humans, were the most recent addition to the Covenant, and served as soldiers and bodyguards within the Covenant military. The Lekgolo are a race of worm-like creatures that can combine to form large, intelligent assemblages called Mgalekgolo, known by humans as "Hunters". Clad in nearly impenetrable armor and armed with their trademark Assault Cannon, they served as the Covenant's heaviest ground troops. Hailing from Balaho, the Unggoy, or "Grunts" as they are known by humans, were the most common and low-ranking caste of the Covenant. They breathe methane gas, and thus must wear a large tank on their back full of methane to survive on other planets. The Kig-Yar, or "Jackals" or "Skirmishers" as they are known by humans, were the scouts and marksmen of the Covenant military due to their superior senses.

The Flood, referred to as the Parasite and the infection by the Covenant, is a species of highly virulent parasitic organisms that reproduce and grow by consuming sentient lifeforms of sufficient biomass and cognitive capability. Originating as a corrupted form of the galaxy's most ancient custodians (the Precursors) the Flood reproduces and spreads by infecting other organisms, hijacking their bodies and nervous systems in order to transform them into one of many specialized forms. Through infecting sentient lifeforms, the Flood assimilates their memories and intelligence, and the species becomes collectively more intelligent as a result. Because of their frighteningly rapid growth rate, as long as enough hosts of sufficient biomass and intelligence are available, the Flood is effectively unstoppable.

The Flood presents a highly variable and unconventional threat in combat, as it can infect and mutate dead or captured opponents into a myriad of deadly forms, effectively growing stronger as it weakens enemy forces. They are widely considered to be the greatest threat to the existence of life (or, more accurately, biodiversity) in the Milky Way galaxy.

The D77-TC Pelican, commonly known as the Pelican, is an extremely versatile space-to-ground capable craft used by the United Nations Space Command, as well as being marketed toward law enforcement, security, corporate and civilian markets.] In its role as a tactical aerospace lifter, the Pelican is mainly for the pickup and transportation of personnel, vehicles and equipment, though some variants are capable of serving in the role of a gunship.

The Kez'katu-pattern Warrior Transport, more commonly known as the Phantom, is a troop carrier and gunship in service with the Covenant. They were used primarily as troop transports, flying into combat zones and deploying infantry while providing suppressing fire.

The Human-Covenant War was instigated by the High Prophets of the Covenant following a discovery which led them to believe that humans were directly related to the Forerunners, contradicting their ancient religion. Seeking to keep various political and religious orders intact, the High Prophets declared a holy war of genocide against the human species, and the discovery of their relation to the Forerunners was kept a secret. With superior firepower, numbers and technology, the Covenant would repeatedly overpower the UNSC across hundreds of human-inhabited worlds, including the heavily defended strongholds of Reach and Earth. Humanity would win only scattered and costly victories.

Requiem, formally designated by its Forerunner creators as Shield World 0001, was a Forerunner shield world located in the Epoloch system. It once served as the central base of operations for the Prometheans, an elite class of Warrior-Servants, for thousands of years until the end of the Forerunner-Flood war. It remained sealed off for a hundred millennia since the end of the war, and became the site of major events as UNSC and Jul 'Mdama's Covenant forces stumbled upon it in 2557. The UNSC Infinity returned to Requiem in early 2558, resulting in a protracted battle between the UNSC and the occupying Promethean-Jul's Covenant alliance.

In the endgame of the Forerunner-Flood war, the Ur-Didact converted his Prometheans into machine-based intelligences, known as Knights, by using the Forerunner machine known as the Composer. These constructs, along with the Ur-Didact, survived the activation of the Halo Array, eventually awakening within Requiem and rising to oppose the UNSC in 2557.