Barney Calhoun is the guard you see knocking on a door in Half Life, the main protagonist in Blue Shift, and a Resistance leader in Half-Life 2 and Episode 1. It is unknown what happens to him by Episode 2.
All the guards of Black Mesa are referred to in the game as "Barney", but this was for generic purposes.
In the HLEU, he's the main protagonist of Sebastian, a mod set during Half-Life 2 Episode 1, where he escapes City 17 through a hospital, all the way stalked by a resilient Strider, only to finally destroy it on the roof.
He's also set to be the main protagonist of Lost Squad, depicting other unknown events during the Half-Life 2 timeline.
In the timeline of the SPY Maps, he is seen to have been killed in Coastline to Atmosphere, and transformed into a Combine soldier in Strider Mountain.