Master Chief Petty Officer John-117, more commonly known as Master Chief, is a SPARTAN-II commando of the UNSC Naval Special Warfare Command. Master Chief is an imposing super-soldier. Without his signature armor, he stands roughly 6'10" tall, and is 7'2" with armor. He has short brown hair, a strong jaw, a firm mouth, serious blue eyes, and unnaturally pale skin from a lifetime of living inside a helmet. Master Chief was born in 2511. He's 41 years old. Master Chief's appearance is defined by the iconic olive green MJOLNIR Powered Assault Armor, featuring a signature gold-tinted visor and a distinct numeral "117" on his shoulder plates.

Cortana is John-117's partner in various combat missions. Cortana was designed to infiltrate computer systems. She can be transferred among various computer systems. She needed a Data Crystal Chip to move in and out of various networks, including John-117's MJOLNIR armor. As Cortana is an AI, she has no physical body. She can communicate through comm systems and project a holographic image of herself from appropriate projectors. Her chosen avatar appears to be a naked tall female human, roughly in her early twenties, with purplish/bluish skin, shoulder length purplish/bluish hair cut shorter at the back, and blue symbols scrolling down her body.

The Covenant are a theocratic hegemony made up of multiple alien species. The Covenant is a political, military, and religious affiliation. The "Prophets" lead the Covenant and exert complete control over all religious and political affairs.

Sergeant Major Avery Johnson serves with the UNSC Marine Corps. He's a broad shouldered, physically fit soldier in his 70s, though his biological age is younger due to extensive cryosleep. Known for his shaved black hair, brown skin and eyes, often seen with a cigar, and a determined and often yelling facial expression. He's 6'1" tall. He primarily wears a green UNSC Marine Corps Battle Uniform.

Captain Jacob Keyes is a highly decorated UNSC Navy officer, best known for commanding the UNSC Pillar of Autumn. Standing at 6'4" and he has a commanding tall stature and wears a gray naval officer uniform/tunic with a name tag, various service ribbons, and a Command Neural Interface.

The Flood is a parasitic alien species that reproduce by infecting hosts with spores or infection forms, which take over the body and convert it into a Flood form. They have various forms.