Better known to the public as the Master Chief, is a SPARTAN-II commando of the UNSC Navy's Special Weapons division. He served as one of the most important figures of the Human-Covenant War. After nearly thirty years of active duty, he is one of the most decorated war veterans of the United Nations Space Command, having fought in more than two hundred ground engagements against the Covenant and Insurrection, earning every major service medal except the Prisoner of War Medallion.

Born Frederic Ellsworth, is a SPARTAN-II commando. As of 2558, he is one of the few remaining SPARTAN-IIs and holds the highest rank among active-duty peers.

Frederic Ellsworth was born on the human colony of Ballast on April 3, 2511. He was abducted from his home in 2517 at age six after he was observed by Dr. Catherine Halsey and marked as a potentially exceptional soldier. He was trained with the other Spartans at Readh by Déjà and Chief Petty Officer Franklin Mendez and participated in numerous training exercises, including Spartans' mission to the  Military Wilderness Training Reserve.

During his training, Fred quickly proved himself to be on of the program's top candidates. Both Dr. Halsey and Chief Mendez identified him as one of the four emerging leaders within the SPARTAN-II group, along with Kurt-051, Jerome-092, and John-117. He was among the Spartans who survived the physical augmentations which enhanced their combat prowess to superhuman levels.

Born Kelly Shaddock, is a SPARTAN-II supersoldier of the UNSC Navy Special Warfare Command. She is renowned for her incredible speed, even without her MJOLNIR armor. As of 2553, she is one of the few active SPARTAN-IIs. She had participated in 201 operations-104 of which were full campaigns-as of 2558, more than any other Spartan with the exception of John-117.

Born Linda Pravdin, is a SPARTAN-II supersoldier. She is one of the closest friends to John-117 and on of the few known surviving SPARTAN-IIs. She is best known for her skills with the sniper rifle and serves as the sniper and scout of Blue Team. By 2558, she had engaged in a total of 189 military operations (of which 174 were full campaigns), most of them during the Human-Covenant War.

A SPARTAN-IV supersoldier and a former Office of Naval Intelligence agent. Prior to becoming a Spartan, Locke had the rank of Lieutenant Commander and served under ONI's Section III as an Acquisitions Specialist, his duties including the retrieval of important objects from the enemy as well as tracking and assassination of high-value individuals.

A SPARTAN-IV supersoldier who served in the UNSC Marine Corps throughout the Human-Covenant War, the last years of which he spent as an Orbital Drop Shock Trooper.
Counting the time spent in cryo-sleep, Buck was a 25 year veteran of the Covenant War and served in numerous battles. He was regarded as an extremely skilled Marine and by the Battle of Earth, he was the leader of a small team, Alpha-Nine, which was appropriated by Captain Veronica Dare for a classified ONI mission before the drop on New Mombasa, Kenya on October 20th, 2552. Following the war, Buck became a SPARTAN-IV and eventually joined Fireteam Osiris, led by Jameson Locke.

A SPARTAN-IV supersoldier of Fireteam Osiris. Vale is an expert in alien culture, particularly the culture of the Sangheili and Forerunners, and is fluent in Sangheili. Following the Human-Covenant War, Vale served as a professional diplomat for the United Nations Space Command, specializing in interspecies relations. While most SPARTAN-IVs are recruited from active field duty, Vale is an exception. Vale's enlistment in the SPARTAN-IV program comes after receiving stellar combat scores in training exercises despite minimal field experience.

A human soldier of the United Nations Space Command who served in the years following the Human-Covenant War. She was one of the three survivors of the 2550 glassing of her homeworld Minab known to have endured the three year struggle on its barren surface. Shortly after their rescue Tanaka would enlist in the UNSC army, becoming a non-commissioned officer in the engineers, and later volunteered for augmentation and training as a SPARTAN-IV supersoldier.

Cortana was a "smart" AI in service with the United Nations Space Command. Unique and exceptionally powerful even among smart AIs, sh was one of the most important figures in the Human-Covenant War, and served as John-117's partner in the final months of the conflict. Her flexibility and tactical skill proved invaluable in boosting the combat ability of th Hayclon-class light cruiser Pillar of Autumn, the orbital defense platform Cairo Station and the Stalwart-class light frigate In Amber Clad. In addition, she held vital data pertaining to the Halo Array, including the activation index of Installation 04.

A key scientific adviser to the Office of Naval Intelligence, best known for her work with the SPARTAN-II program, developing and creating smart AIs, and for overseeing the development of MJOLNIR Powered Assault Armor. She served as a scientific adviser for ONI from 2515 to 2522, when she was made chief scientist, a position she held until the end of the Human-Covenant War.

Halsey held considerable authority within ONI, despite being a civilian, and commanded respect from important military figures. Though generally well-regarded among her peers, she endured intense personal guilt for condemning the Spartans to the life of a super soldier. After the Covenant War, Dr Halsey was stripped of her position, incarcerated, and treated as a war criminal by the UNSC at large, which led to her apparent defection to Jul 'Mdama's splinter Covenant faction.

The Sangheili kaidon of House of 'Vadam. He was the last Arbiter to serve the Covenant, and continued to be widely known by his title in the closing months of the Human-Covenant War and in the post-war period. Presently he leads the Swords of Sanghelios and is considered by many to be the formal leader of the Sangheili on the whole.

Thel commanded numerous fleets during his service in the Covenant, most notably the Supreme Commander of the Fleet of Particular Justice, which mounted a successful invasion and occupation of the human colony Reach and later during operations on Installation 04. 'Vadam was responsible for over 1 billion human casualties during the war, and the glassing of at least seven colonies. During the Great Schism, Thel became a leading member of the Sangheili who seceded from the Covenant, and aided the UNSC in their effort to stop the Prophet of Truth and his forces from activating the Halo Array. He fought many battles alongside the "demon" John-117 on Installation 05, Earth and Installation 00 in the final days of the war.

Returning to Sanghelios after the conclusion of the war, the Arbiter went to become a symbol of Sangheili honor and resilience in the face of betrayal. He attempted to unite the feudal states across his homeworld in forming the first coalition since the First Age of Conflict, despite his efforts, he met resistance from many of his people, who felt his ideals and beliefs went against the traditional values of the Sangheili.

A senior officer of the UNSC Navy who served throughout the Human-Covenant War and, in the post-war era, played a key role in the conflicts with the Covenant remnants and Forerunner Prometheans. Since at least 2552, then-commander Lasky served as Captain Del Rio's executive officer aboard the UNSC Infinity until July 2557, when Del Rio was replaced as captain by Lasky just prior to the Promethean assault on Earth.

Known to his followers as the "Didact's Hand" or "Hand of the Didact", is a Sangheili shipmaster previously in the service of the Covenant during the war against humanity. Fueled by hatred for humanity after the death of his wife, 'Mdama became the supreme leader of a resurgent Covenant faction in the wake of the Great Schism. Despite years of eluding the public light, his activities on the shield world Requiem  established him as a high valued target for the United Nations Space Command.

Mahkee 'Chava is a Sangheili in service with the Swords of Sanghelios. In 2558 Mahkee aided Fireteam Osiris with their hunt for John-117 and the rest of Blue Team.

Is a Forerunner Promethean construct that considers itself the "keeper of the Domain and her secrets". During the Battle of Sunaion, the Warden Eternal was encountered by Fireteam Osiris and incapacitated them in battle with a hardlight blade. It then informed Spartan Jameson Locke that John-117 had been granted access to a nearby Guardian, though such permission was denied to Locke and his team.

A soldier in the SPARTAN branch of the United Nations Space Command Defense Force and a veteran of the Human-Covenant War. After the war, Palmer transferred from the UNSC Marine Corps Shock Troopers to the newly-independent branch, and was augmented as one of the first SPARTAN-IN supersoldiers. She is currently assigned as the Commanding Officer of the 300-member Spartan contingent aboard the UNSC Infinity.

Roland is an artificial intelligence construct in service with the United Nations Space Command. He is the primary AI of the UNSC Infinity. His name and avatar are based on Roland Bearmont, a British fighter and test pilot for the Royal Air Force during and after World War II.

Entering service on December 5, 2557 on Earth, Roland replaced Infinity's former shipboard AI Aine, who was destroyed when the ship crashed on Requiem in July 2557.

A SPARTAN-IV supersoldier who served with Fireteam Majestic on the UNSC Infinity in 2558.

Grant was born on November 4, 2533 on Arcadia, which been briefly invaded by the Covenant two years earlier. During her childhood, she was friends with Jordan Gaines. On September 9th, 2549, when she was 16, she was on Arcadia during the planet's fall. Grant joined the UNSC Navy at a young age and was later accepted into the SPARTAN-IV program after its inception. Her athleticism and combat skills gained her placement in an operational division in which she field-tested early versions of MJOLNIR GEN2. She was soon given an active role as a Spartan and was eventually assigned to Fireteam Majestic.

A SPARTAN-IV supersoldier who served with Fireteam Majestic on the UNSC Infinity in 2558.

Anthony Madsen was born October 30, 2530 on New Llaneli. He served an undistinguished career as a marine before being admitted into the SPARTAN-IV program because of the recommendation of his father and his grandfather, both being Generals in the Marine Corps.