According to Variety, Agent Locke is played by Mike Colter, an actor best known for his roles in American Horror Story and The Good Wife. Plot details are scant, but during the course of Halo: Nightfall, Locke is reportedly sent to a "distant colony world" to investigate terrorist activity on behalf of the Office of Naval Intelligence. There's more to this story than Locke initially suspects and while investigating the colony, he discovers an ancient alien artifact that kicks off the series' major plot points.
Beyond his appearance as the lead in Halo: Nightfall, Locke will also appear in the upcoming Xbox One-exclusive Halo 5: Guardians. It's currently unknown what role he'll play in the game.