Which Halo game first introduced the playable Spartan Locke campaign perspective?

Halo 5: Guardians first introduced the playable Spartan Locke campaign perspective. Players control Spartan Jameson Locke and Fireteam Osiris for most of the game, while also playing missions as Master Chief and Blue Team.

Locke is an ONI operative and Spartan-IV who leads Fireteam Osiris. The team is sent to pursue Master Chief after he leaves the UNSC chain of command to search for Cortana. This creates a campaign structure built around two Spartan teams and two parallel viewpoints.

Halo 5 was released for Xbox One in 2015 and used four-player online cooperative campaign play. Each player could control a member of the active fireteam, although the story’s named central perspective changes by mission. Locke later appears in the Halo television series, but the game character and the television adaptation belong to separate continuities.