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Black-One (name and Spartan tag unknown) is a SPARTAN-II Commando of the UNSC Naval Special Warfare Command and the leader of Team Black. Her visor is etched with the Roman numeral "I".

Conscription
She was abducted and conscripted into the SPARTAN-II program by Dr. Catherine Halsey in 2517 when she was six years old. She was then taken to the planet Reach, and trained by the AI Déjà and Chief Petty Officer Mendez, along with the rest of the Spartan children. She was assigned as the leader of Team Black, and trained with the other teams such as the Red Team. In 2525, she and the other candidates underwent a series of augmentations to improve their abilities. Black-One was among the ones that survived and were not crippled or killed by the procedure.

While training, Black-One was in a secret relationship with Black-Three. Black-Four was jealous of Three and ratted them out to Chief Mendez, who sent them both to the brig. Red Team tracked Black-One down as team leader and attacked her, considering such "childish" things like love unnecessary in becoming a Spartan. Black-One lost her left eye resisting them.

Later Career
During the Human-Covenant War, Black-One served in the Battle of Verge and a mission on an Unnamed Moon. She was captured by mechanical collectors, who brought her, among other Covenant soldiers to an alien complex, where an alien Monitor named 686 Ebullient Prism was going to dissect all of them to find out if any of them were immune to the Flood. Luckily, Black-One was able to free herself and an Elite named Reff 'Talamee by using her helmet to deflect a laser at one of her ammo magazines lodged in one of the collectors. She then grabbed the robot's weapon to use against Ebullient Prism. Once thye had avoided and blinded the Monitor, the two discover a complex piece of machinery.