Summary
Commander Miranda Keyes (voiced by Julie Benz) is the daughter of Captain Jacob Keyes. She is never even mentioned until Halo 2. As of 2552, she held the rank of Commander in the UNSC navy, and commanded the frigate UNSC In Amber Clad.
That year, she accepted the posthumous Colonial Cross for her father for his actions during the events of Installation 04. Her vessel was docked with Cairo Station during The First Battle of Earth. She fought her way to her vessel with Sergeant Johnson and took her ship down to New Mombasa, where she dispatched Pelican dropships full of forces to join the massive battle with the Covenant in progress there. One of those Pelicans carried Johnson and the Master Chief.
During the battle, the In Amber Clad was the only vessel able to follow the High Prophet of Regret when his Assault Carrier entered slipspace inside the city. They followed the Carrier and ended up at Installation 05. There, she dispatched her full compliment of ODSTs to assist the Chief in his pursuit of the prophet of Regret. She then ordered him to kill that Prophet.
Meanwhile, Miranda and Sergeant Johnson went to the Library of the Halo along with most of the Marines of the ship. They attempted to retrieve the Index and although Keyes was sucessful, most of the Marines were killed or infected by the Flood, and Keyes herself was captured by Tartarus and his band of Brutes. The Brutes then took Keyes and Johnson to High Charity, where they met the High Prophets of Truth and Mercy. They were taken aboard Phantom dropships before the Master Chief could rescue them.
Keyes was then taken to the Control Room of Installation 05 because she had been designated a Reclaimer by 343 Guilty Spark. Tartarus tried to force her to activate the Installation, but they were interrupted by the Arbiter and Johnson, who killed Tartarus. She then questioned Spark about the Halos and was told that they were on standby and could be remote activated at The Ark.
Her current whereabouts are unknown, but it seems she is somewhere on Installation 05 and in the company of the Arbiter, Johnson, and Spark.
Gameplay
Like Sergeant Johnson and other NPCs of storyline importance, she cannot be hurt by any available means. It is possible to test this out in The Great Journey (Level). By utilizing a glitch involving sticking Plasma Grenades to her, it is possible to push her down to the structure where Tartarus lurks. Despite his best efforts, she is invulnerable to his onslaught.
Speculation
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. It is safe to assume that Miranda's mother is Dr. Halsey.