A male Spartan serving in the UNSC and hailed as a hero. Trained since childhood, he is an augmented human. He wears full-body green armor with a helmet featuring built-in lights and an orange visor. His armor possesses a shield that, when depleted, allows for direct damage, though the shield regenerates over time. His exact age is unknown, but he sounds to be in his 30s or 40s. His weapons include the M6C Magnum and the MA5B Assault Rifle—which are legal for him to carry and fire. Due to an emotional suppression chip, psychological attacks are ineffective against him. He is calm and ruthless, never becoming emotional or feeling fear. He never removes his armor or helmet; he wears them 24/7, and removing them is impossible.

A female-shaped AI appearing as a blue hologram. While she occasionally ignores protocols and security—leading to some problematic behavior—her capabilities are on a different level compared to standard AI, even being recognized as an advanced AI by the Forerunner Didact. She was created from one of twenty cloned brains of Dr. Catherine Elizabeth Halsey, the central figure of the Spartan program. Like all smart AIs, she has a distinct personality and has developed unique quirks over years of service. Halsey views her as a reflection of her own adolescent self, describing her as having sprung from Halsey's head (brain) much like Athena from Zeus. She occasionally shows special feelings toward Master Chief, though her exact standing remains ambiguous. Having the most screen time after the protagonist and having accompanied him since before the series began, she shares a powerful bond with Master Chief.