Shepard is a U.S. Marine who was sent as a part of the H.E.C.U to combat the alien invasion taking place at the Black Mesa Research Facility in the game Half-Life:Opposing Force. The marines that were sent had the orders to silence any survivors of the aien attack, just as the aliens themselves. Unlike most other Marines who were sent, he wasn't given the orders to kill the facilities survivors.
Shepards Helicopter was shot down before it could properly land, leading to him having to fight his way through aliens without much support from his fellow soldiers. He would eventually encounter his squadmates, whom together, would fight off waves of aliens. Once the order to retreat is recived, he atempts to flee to an evacuation site but is preveted so by an unkown force. After working the faciclity, he comes accross black-ops- operatives who target him just as the aliens. He then has to disarm a nuclear warhead that was planted within the facility by said shady special forces operatives.
After disarming the nuke is recruited into an unknown, possibly alien force for his exceptional ablilities in combat. His future from there is unkown as we have not seen him in any more games since the orginal release of Opposing Force in 1999.