Master Chief

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.

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.

The MA5C Individual Combat Weapon System, abbreviated MA5C ICWS and commonly known as the MA5C assault rifle, is a standard-issue service rifle for the UNSC Marine Corps and the Navy.

The M7/Caseless Submachine Gun (M7/SMG) is a personal defense weapon developed and manufactured by Misriah Armory.

The M6G2 Personal Defense Weapon System (PDWS), or "Magnum", is a variant in the M6 series of sidearms produced by Misriah Armory.

The BR55HB SR is employed as a medium-to-long range marksman rifle. It fires M634 X-HP-SAP round from a 36-round magazine, which fits flush in the receiver. The magazine housing is built directly into the underside of the stock of the rifle and is located behind the grip. Though the BR55HB SR is a select-fire weapon, it is most often used in its three-round burst mode.

The M90 Close Assault Weapon System (CAWS), also known as the M90 Close Assault Shotgun, M90 assault shotgun, or simply the M90, is a pump-action shotgun in service with UNSC.

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 Pod infector also more commonly known as the Flood infection form, is a stage of the virulent Flood xenoform. It is the most common Flood form and is usually the first forms to be released. It is the primary form for spreading the Flood infection due to its effectiveness compared to mere Flood spores when deployed in large numbers.

While small (roughly the size of a human torso) and weak, they travel in huge swarms and overwhelm foes using sheer numbers. Once within range of a potential host, they leap for the victim's chest area, rasping away at armor, clothing and flesh with their numerous tentacle-like limbs. These appendages penetrate deep into the body, tap into the victim's spinal cord, and unleash an attack on the host's nervous system via direct contact with the spine. Once this is accomplished, the form rewrites the neural pathways of the victim's brain with its tendrils, forcing a resonant frequency match between its neural signals and the host's. At this point, the Pod infector has complete control over the body's motor functions.

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.