The mysterious character seen in the Halo 5: Guardians teaser image above is named Agent Locke, and he'll be featured in acclaimed director Ridley Scott's upcoming digital Halo series.
Locke's identity was revealed to Polygon by 343 Industries head Bonnie Ross, who said he'll have "a key role in Halo 5: Guardians." That much was to be expected, given that the blue-visored Spartan was featured opposite the Master Chief in the Halo 5 teaser image released last month. Microsoft and 343 avoided saying who he was at the time, only confirming that it was a new male character, not Cortana.
Mysterious as Locke remains for now, we won't have to wait until Halo 5 is released next year to learn more. "We're using Halo Nightfall to basically tell his backstory and tell his origin story," Ross said.
Nightfall is the episodic digital series from Scott that Halo: The Master Chief Collection buyers will get full access to. It was reported just last week that filming was underway in Ireland. Ross confirmed filming has wrapped up in Ireland, with Iceland next on the schedule. She also said it's not mandatory viewing: "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," she explained.
The Master Chief Collection will include more new story content than just Nightfall. Hidden terminals are being added to Halo 2, and by finding them, you'll be able to watch animated stories that follow the Arbiter's story. Additionally, a new prologue (taking place before Halo: Combat Evolved) and epilogue (taking place between Halo 4 and Halo 5) are being produced to further flesh out the universe.
Microsoft confirmed rumors this week when it announced The Master Chief Collection during its E3 press conference on Monday. The collection includes all four Halo games to star the Master Chief and, as we've since learned, will offer new skulls, as well as saved films and Forge support--but only in the games that originally supported them.