Catherine Halsey

Although technically a civilian consultant (#409871) with Civilian Identification Number 10141-026-SRB4695. She, a biological scientist, had an M.D. and Ph.D.

The SPARTAN-II Project
Dr. Catherine Halsey was the person responsible for both the SPARTAN-II project, and the MJOLNIR Mark V armor. She, along with Captain Jacob Keyes (who at the time was a Lieutenant), handpicked the first candidates for the Spartan program, and brought them to Reach.

On Reach, she trained the Spartans along with Chief Petty Officer Mendez and the AI Deja. She looked over their augmentation procedures, combat training, and studied the effects of the Spartan lifestyle on the children. She kept in close contact with them all throughout their lifetimes, from her base under Reach.

Fall of Reach
After the Covenant's invasion of Reach, she volunteered to remain at ONI's CASTLE Base to destroy all the data that ONI had left behind. But fortunately, surviving Spartan soldiers found her and they were able to escape with the Forerunner Crystal ONI had been protecting.

Escape into the Unknown
She later saw the importance of destroying this artifact, for it was too dangerous and uncontrolable to use. After escaping Reach, she stole a small spacecraft and, along with Kelly-087 sedated, went into slipspace. Her whereabouts was unknown until the events of Ghosts of Onyx.

The Dr. returns
In Ghosts of Onyx, Halsey and Kelly eventually make it to Onyx, while reuniting with Chief Petty Officer Mendez and eventually her remaining SPARTAN-II soldiers. She helps decipher the mystery of the planet.

Halsey's Guilt
Halsey carried extraordinary guilt because of the SPARTAN-II project, and she felt that she had exploited the children and destroyed their lives. Though she continued to support them and gain monumental prestige for her efforts, she also felt responsible for each one who died. She never could quite justify their exploitation as a "necessary sacrifice". This likely accounts for her actions in abducting Spartan-087.

Theories
Due to Miranda's similar physical apprearance with Cortana, fan-fics have been written that Captain Jacob Keyes (when he was a Lieutenant Junior Grade) and Dr. Halsey had a brief romance at some point. This is highly possible since the first chapter of Halo: The Fall of Reach showed that Keyes was attracted to her, as well as various other hints. It is therefore not unreasonable to assume that Miranda's mother is Dr. Halsey.

Appearances

 * Halo: The Fall of Reach
 * Halo: First Strike