From Halopedia, the Halo wiki
The individual known by the callsign "NV66" was trained from a young age by Senior Chief Petty Officer Franklin Mendez to be a Spartan, an extraordinary form of warrior created to serve the United Nations Space Command Navy. When they were in their teenage years, this person underwent a series of biological augmentations to perfect their physical abilities. In July or August of 2552, NV66 was on Reach in the midst of the fight against the Covenant there. NV66 was part of a Spartan team which also included one identified by the callsign "NOLA". This team was tasked with the protection of important Office of Naval Intelligence Section Three intel that was under threat of acquisition by the alien menace. The intelligence concerned research and technical specifications having to do with improving MJOLNIR Powered Assault Armor. The team of Spartans managed to securely transmit the information using an encrypted data node to a remote archival station before coming under heavy fire from the enemy. NV66 and the rest of the team were declared missing in action following this engagement. The data they sent out was thought to have been lost forever along with the rest of the planet, but in 2556 it was recovered and Section Three operatives subjected it to analysis. It appeared to them that the key to unscrambling the heavy encryption of the data had been generated from some of the Spartans involved in the operation to keep it safe back in 2552.