His name is Agent Locke.
It almost feels like we took to long to find out, but Microsoft has finally revealed the name of our would be deuteragonist for Halo 5: Guardians.
Microsoft has also revealed that Agent Locke’s story will be revealed in Halo Nightfall, the special series made by Ridley Scott. Halo Nightfall will itself be available to people who purchase Halo: The Master Chief Collection.
You can get some more details on Locke and Halo Nightfall here.