The mysterious spartan playing a major role in Halo 5: Guardians is Agent Locke.
The soldier's story will be told in Halo Nightfall, the episodic series made by Ridley Scott to go with the game, as explained by Polygon.
The Spartan with a blue visor appeared on the first image for the game, when it was officially detailed last month. Locke will play "a key role in Halo 5: Guardians," general manager and 343 Industries studio head Bonnie Ross said.
"We're using Halo Nightfall to basically tell his backstory and tell his origin story," Ross said during a Halo-themed meeting at E3 2014. "You don't have to watch this, but knowing this will kind of tee you up for all the new things that are going on in Halo 5: Guardians."