Barney Calhoun is a security officer assigned to the Area 3 Security sector of the Black Mesa Research Facility. He is a very good friend of Gordon Freeman (whom he owes a beer), as well as Isaac Kleiner and Eli Vance.
Barney Calhoun appears in Black Mesa in a manner almost identical to the original Half-Life; his identity and location is not officially canonized at any point in the game, allowing his presence to be almost entirely up to the interpretation of the player.
The only in-game NPC that is subject to this interpretation will be a particular security guard seen in the Inbound chapter. His face is not visible, and the player has no means of interacting with him; he lacks the work-dress and is attempting to gain entry to a security block. This was a deliberate choice on the part of the developers to avoid unnecessary conflict between the narratives offered by Half-Life and Half-Life: Blue Shift, the latter of which portrays this guard as Barney Calhoun.
- If the player uses the "noclip" cheat to get to Barney, one will find that he is just a generic security guard using the Face Creation System, and isn't a reused or reskinned Barney from Half-Life 2.