Cortana is the A.I. program that was once Catherine Halsey. Doctor Halsey programed her brain into a single disc of knowledge. Cortana is that knowledge, and she is the most important material in the UNSC army.
Cortana is the A.I. (artificial intelligence) that was created by Doctor Catherine Halsey. Doctor Halsey was the founder of the Spartan project, a UNSC (secret) project that would enhance any "normal" human beings physical senses to the max. However, the project would take hard and enduring training that would sometimes, kill the subjects. Catherine Halsey wanted to abduct children, so that they can be trained to be military, however, she didn't want the parents (of the child) to be without their daughter or son. So she cloned the child, and gave the clone to the parents, the parents didn't know of what happened to the original. Cortana is supposed to be the smartest A.I. in the galaxy. Other A.I. are (supposed) to be called dumb, because of their lack of emotion and idealistic of just getting the mission complete, and nothing else. Cortana on the other hand has emotions. She is in pain at times, is funny, and has other (humanistic) character traits. This meaning that she was the brainchild or clone, of Doctor Halsey's brain.
Doctor Halsey created the artificial intelligence (named Cortana) when reach was overrun by the Covenant. This was a "last" moment thing, and she knew that she wasn't going to live forever. So Halsey scanned her brain, and then made a small "carry-on" microchip that would fit into the helmet of a Spartan. The A.I. can only work on Spartan-II programing, as it is the only programing that will work.
Once Cortana was created, Doctor Halsey and her new A.I. left Reach with the UNSC army. After traveling from the (now Covenant infested) planet known as Reach, the Covenant then destroyed the planet. Reach was the earth's first (and last) defense against an alien invasion. Days after Cortana's creation, Doctor Halsey gave the A.I. a choice of 30 spartans to only pick one. Out of all 29 spartans, Cortana picked Master Chief. Master Chief and Cortana bonded like a brother and a sister. The chief is the only Spartan that Cortana "fully" trusts.
The planet Reach was Earth's last defense against an alien invasion. Once Reach was destroyed by the Covenant, the alien scum had to go to the planet Earth to find their "alien death machine". Before Reach was destroyed and before, Master Chief took up the fight against the Covenant, Harvest was the first planet. Harvest was our (first) defense against an alien invasion, however, due to poor skills of planing and bad timing, the Covenant destroyed that planet also.
When the Covenant arrived on Earth, Lord Hood (the supreme commander) ordered Master Chief and his band of marines arrived in a abandon bunker. When the Chief walks out, he is greeted by a small army of Elites. Cortana guides him to different towers, that will allow him to activate certain and missionary objectives. Master Chief and Cortana make an unbeatable team!
When "Halo" is brought up, Cortana has to explain to Master Chief what it is. Halo turns out to be a "forerunner" death machine that would destroy "all" organic matter, and the covenant was trying to activate it so that everyone can go to alien heaven! So when Master Chief tells Lord Hood and the rest of the UNSC, Hood decides that they have to decommission all of the 9 rings. 343 Guilty Spark joined Cortana and the UNSC on this mission of destroying the rings. Throughout Halo Combat Evolved and Halo 2, the UNSC and Master Chief, deactivated most of the rings. When an elite (the arbiter) spoke against the covenant and the prophet, he was given the "mark of shame" and was stripped of his ranks and made as a Arbiter. During these events, Arbiter was having second thoughts about joining the UNSC, however his older brother, wanted him to stay with the "great journey". In the end, him and his brother joined the UNSC. When Master Chief was confronted by a beast called, Gravemind, the monster also took Cortana and sent the Spartan back out of the water, with the Arbiter.
After two or more years of constant battle, Master Chief's and Cortana's fight for all humanity and universe was about to come to an end. Humanity (and the UNSC) decided to join with the rebels of the Covenant army. Arbiter, Shipmaster and a whole bunch of other elites joined them and the UNSC to protect the entire universe. Master Chief kept having visions of Cortana calling from (the base of Gravemind), in pain. So Chief and Arbiter would soon get the mission of finding Cortana so that she can once again help the human race win the ultimate fight. When Arbiter, Master Chief, and Cortana were on the final ring, they destroyed it and escaped into a giant ship that Miranda Keyes was once driving. All three of them escaped, however, only Arbiter's part of the ship made it towards earth. Master Chief was stated M.I.A. Cortana and Master Chief were stuck in space, with only half of a ship, she sent out the beacon, however, Cortana said it could take years for them to be found. Master Chief climbed into a sleeping pod, and told Cortana to, "wake me if you need me". Master Chief's journey and fight for humanity finally ends. The Chief gets some well deserved rest, however the fight doesn't end. Because the ship ends up being pulled into Requiem's orbit.
A collection of anime shorts based on the popular Halo franchise of video games.