He is a Corporal in the U.S. Marines, assigned to the Hazardous Environment Combat Unit, an elite special forces detachment of the Marines sent to Black Mesa to simultaneously thwart an alien invasion and silence any surviving personnel.
In the manual for Half-Life: Opposing Force (as well as, oddly, Half-Life: Blue Shift) there are journal entries by Shephard that give insight to events before the Black Mesa Incident.
- "May 3 - 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 civvie 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."
- "May 7 - 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..."
- "May 9 - 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 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..."
- "May 12 - 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..."
- "May 15 - 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."
Portal Easter Egg
While playing through Portal the player will eventually come across several rooms containing lots of computers and keyboards. The only keys on the keyboards that are highlighted yellow are the letters that spell out "Adrian Shephard" (A, D, E, H, I, N, P, R, S).