Cortana is an Artificial Idiocy unit that stars in the Halo shames, and is also the mommy of Master Chief. Nobody knows how AI units can have babies. I don't even know how regular people can have babies. But Cortana is Master Chief's mommy. Also, she died in Halo 4ever. Oh yeah, spoiler alert.
Cortana was once a brave warrior of Sparta. And by warrior, I mean boring loser who filed paperwork. But then she had to pick a spartan to adopt. She chose the Master Chief because HE'S THE MASTER CHIEF! Then they went on lotsa adventures together. Master Chief did all of the work though. All Cortana did was boss Chief around give him "intel", which is short for "not intelligent".