Cpl. Adrian Shephard is the protagonist of Half-Life: Opposing Force, a US Marine Corporal assigned to the Hazardous Environment Combat Unit from the Santego Military Base in Arizona, where he was trained by Drill Instructors Dwight T. Barnes and Sharp. During the Black Mesa Incident, he is one of the men sent to the Black Mesa Research Facility to kill the Xen invaders and later silence the witnesses, especially a scientist named Gordon Freeman, as part of a classified mission. Early in the game, Shephard finds himself separated from his unit and ends up fighting solely for his life, aided by fellow Marines scattered around the facility and Black Mesa personnel, and in addition to the Xenian invaders confronts another alien faction, Race X, and the Black Ops, who become much more prominent enemies during the events of Opposing Force in comparison to the other games in the series. He was 22 years old at the time of the events. Shephard was to reach Black Mesa aboard an Osprey Heliplane designated Goose 7. With him were other soldiers such as Tower and Jackson. When the nearby Osprey, Goose 3, and his are attacked by Alien Crafts, it crashes in Black Mesa, killing most of the passengers. Shephard must then make his way through Black Mesa to be evacuated. When he arrives at the extraction point, however, the G-Man prevents him from catching the last Osprey, forcing Shephard to leave the facility by other means and going through parts of the facilities not seen in Half-Life and dealing with the Race X aliens and the Black Ops, to finally defeat the Gene Worm.

Unlike the other Marines, Shephard works with the increasingly distrustful scientists and guards in order to make it out of Black Mesa alive. This is due to the fact that he never received the orders to kill them, as his Osprey was shot down before his commander could inform the squad of their mission.