From Halopedia, the Halo wiki
In Halo: Combat Evolved, Johnson can appear in multiple levels as a standard allied Marine. In Halo 2 and Halo 3 Johnson cannot be killed. In Halo 3: ODST Johnson is a playable character for Firefight.
Halo Wars 2
Sergeant Johnson can be purchased at the UNSC Armory as a hero unit. Johnson is able to heal friendly units within an area with his Y ability, Repair Beacon. Un-upgraded Johnson performs well against vehicles, but only okay against infantry, aircraft, and structures.