It looks like Master Chief won’t be the only playable character in Halo 5: Guardians, as it has now been confirmed that Agent Locke will be playable when the game launches in 2015, as revealed during a San Diego: Comic-Con panel.
Locke is the recently revealed star of the upcoming Halo: Nightfall, the live-action feature that will be included in the Master Chief: Collection. Nightfall will lead directly into Halo 5, and presumably set Locke up to be a major player in the game.
Whether Locke will have abilities separate of Master Chief similar to the Arbiter in Halo 2, is currently unknown.
Also at SDCC, 343 revealed a first look of Halo: Nightfall and Locke’s role within it.