"What's your name, dirtbag? Sound off like you got a pair! Corporal Shephard, huh? Looks more like Corporal "Dog Meat" to me! Seems your name was mysteriously bumped to the top of the advanced training list!"

―Dwight T. Barnes[src]

Corporal Adrian Shephard is a US Marine Corporal[1] assigned to the Hazardous Environment Combat Unit from the Santego Military Base, Arizona, where he was trained by Drill Instructors Dwight T. Barnes and Sharpe, and the protagonist of Half-Life: Opposing Force.

He is one of the marines sent to the Black Mesa Research Facility. Shephard, however, is not aware of his classified mission to silence the witnesses, especially a scientist named Gordon Freeman, since his squad leader dies before he can be briefed. Soon Shephard finds himself separated from his unit and ends up fighting solely for his life against the invading Race X, aided by fellow marines scattered around the facility and Black Mesa employees.

The Opposing Force instruction manual features several diary entries made by Adrian Shephard prior to the Black Mesa Incident, giving some insight of his Advanced Training before the disaster and G-Man's interest in him.

• "March 3rd — Another typically hellish day at base camp... I'll be glad when this is over and I can finally can get assigned a mission. There has been this really weird civie spotted at the base. Rumor is he's from some government branch looking to recruit; others say he's with some secret research group. I would jump at the chance to join. It would be cool just for the change and the adventure."

• "March 7th — I finally saw the government guy today. I am not sure he is a g-man, but he was wearing a really uptight suit and carrying a briefcase. He looked more like a lawyer or insurance agent to me. I did notice him checking me out. Several times throughout the day I spotted him just watching me during training. I wonder what he's up to..."

• "March 9th — For weeks our drills have been the same crap day after day. Today we assemble for the morning run and our drill instructor tells us we have one week to become experts at indoor strategic combat. We will be spending every day this week at the combat simulation facility. As far as I know this is a specialized training not taught in boot camp. What I want to know is if this is to test our ability to adapt or if we are being readied for a specific mission? Time will tell..."

• "March 12th — The rumors have been flying since our indoor combat training began. Most of my peers are convinced that we are being primed for a mission. No one can agree on what the mission is. I have heard the name Black Mesa Facility thrown around a lot, but I have no information about the place. The rumors are that some top-secret research is going on there. Doesn't sound too exciting to me..."

• "March 15th — The rumor has been confirmed. We are being trained for a mission at the Black Mesa Facility. All I know is that the place is being used by scientists who are doing some kind of new research. I can't imagine what we would be needed for. We were told today to be ready in case it happens tomorrow. I don't know what "it" is, but the whole thing is a little strange. I kind of hope it doesn't happen; the mission doesn't seem to have much excitement potential. I'd rather hold out for something with more likelihood of combat."