Dr. Catherine Elizabeth Halsey
'Dr. Catherine Elizabeth Halsey or Dr. Halsey is a well known scientist in ONI. She is seen to be the mother of Cortana and the SPARTAN-IIs as these were her two biggest programs. She has also worked on the MJOLNIR Power Suits. Although Halsey is a citizen, she has a lot of authority in the ONI.
When Halsey was 25 she and Jacob Keyes went looking for 75 candidates for the Spartan program. During these trips she fell in love with Keyes and had given birth to Miranda Keyes. Halsey ended up finding 150 people that could fit into the program but due to insignificant funding she was only allowed 75. Once she had choose the children they were brought back to Reach. The children were replaced with flash clones.
 Early Reach
On Reach, Halsey would supervise the children during their training, whether it be their Education, Combat Training or Augmentation Procedures. She would keep in touch with them from her base under Reach, she could also tell all of them apart even under there armour and would always call them from their real name. She also knew many of their secrets including the security tune, Oly Oly Oxen Free.
 Fall of Reach
When Reach was falling, she called Noble Team about how the Elites were there to collect data. She would later go meet with Master Chief after he was debriefed, she would also meet with the other Spartan IIs. When all looked lost, she called Noble Team to deliver a package to the Pillar of Autumn. The Package was Cortana and she entrusted it to Spartan-B312.
Halsey left with Jun-A266 and it is unknown where they went. Later she went to Castle Base to destroy the remaining AI. While she was there she was met by Spartans Fred-104, Kelly-087, Vinh-030, Isaac-039, and Will-043. They had enough firepower to fight there way out. After realising she would have to destroy the AI. She read through Colonel Ackerson's note's. These note's spoke of Onyx, once she knew where Onyx was she activated CASTLE base's self destruct mechanism, she then led the Sparatans to the mineshaft. Whilst looking through the Mineshaft, Fred found a Forerunner Crystal which sent an energy strike out and attracted Covenant forces to their presence.
During the Covenant Attack Halsey lost contract with Isaac and Vinh. They were then stuck in a hallway for a few days until Master Cheif and the remaining Spartans came to rescue them. They took them to the captured Ascendant Justice. After escaping Reach, the group fled to an asteroid belt, where the Spartans first Mission was held. Halsey then sedated Kelly and kidnapped her in a rebel ship.
In the aftermath of the Battle of Reach, the UNSC made a plague for her life due to the fact that non of the Spartans told of her escape.
 Battle of Onyx
While on their way to Onyx, Kelly and ‘’’Halsey’’’ were reunited with SCPO Mendez and the SPARTAN-IIs and SPARTAN-IIIs. At Onyx she discover the mystery of the planet, at first her intentions were kept secret, but after Kurt-051 confronted her about it she admitted she wasn’t looking for weapons but the Forerunner Shield World. Which she though was a bomb shelter, which would keep all Spartans safe until the war was over.
During the battle it was vital to the survival of the UNSC forces on the planet. After the battle along with Mendez, Blue Team and the rest of the Spartan III survivors they discovered the Shield World. They became trapped there, the fate of them is not currently known yet.