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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.

Training
While training, Black-One was in a secret relationship with Black-Three. Victor-101 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. Black Team later got payback, resulting in another ten days in lockup.

Later Career
During the Human-Covenant War, Black-One served in the Battle of Verge and a mission on an Unnamed Moon. For the latter mission, the UNSC Long Time Coming was transferring Black Team in cryo-stasis when they were shot down on the unnamed moon. The ship's AI, Iona woke the Spartans from their slumber. Black-Two was then assigned to remain on the Long Time Coming while the rest of team goes to investigate a nearby structure containing breathable atmosphere. Unfortunately, they were ambushed by the Covenant, who were also shot down on the moon. Black-One 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 Gatherers. She then grabbed the robot's weapon to use against Ebullient Prism. Once they had avoided and blinded the Monitor, the two discover the Installation's central control center. Accessing the console, One learns that a Covenant fleet was in proximity to the moon. She tries to contact the rest of her team but Black-Three and Victor were engaged in a brawl, having been manipulated by Prism into fighting each other. To stop this, One tells Victor that she forgives him, giving Three the opening to jack in Iona's Data Crystal Chip into Victor's MJOLNIR armor, causing an influx of the team's various mission to remind Victor about his family and bring him out of his tantrum.

Then suddenly, One is slammed into the console by a Hunter and knocked out. Reff tried defending to his brother Thon that the Spartan saved him. Thon snaps that he honored the truce he made with the Spartans only to rescue him. He then changes the subject to inquiring how to contact the holy city of High Charity. However, Reff stated that they should instead blow the servants of the Prophets out sky, having developed a delusion that his kidnapping was the work of destiny. He then killed Thon when his brother attempted to kill him for speaking nonsense. Reff was then vaporized by Prism. Black-One managed to recover to assist the rest of her team in stopping Prism, directing him into the path of the Installation's main gun, vaporizing the Monitor while simultaneously sending the Covenatn fleet in Slipspace transit crashing down to the moon. Black Team then began to fight through the surviving Covenant to commandeer a functional Slipspace drive.