Joe

Joe was a member of the Resistance who manned one of the Underground Railroad stations. He appears in the Route Kanal chapter of Half-Life 2.

Biography
Joe lived with a Vortigaunt in a secluded, hidden little Underground Railroad outpost which was really a boxcar. This concealed them from all Combine (and probably unwary refugees as well), as the only real way in from the outside was a hole in the top.

One day, Gordon Freeman fell into the room from the hole in the ceiling. Joe did not recognize him at first, but after his Vortigaunt companion confirmed that it was Freeman, and added that the "Combine's reckoning had come", Joe allowed Freeman to make use of their supplies. Joe looked at the Citadel and noted how unusually full of activity it was, and warned Freeman that if the Combine caught him down there, it could cause the destruction of the entire railroad.

Joe's Vortigaunt ally then used a jolt of energy to repair Freeman's HEV Suit, and Joe gave Freeman directions to the Underground Railroad's Main Station.

G-Man
The G-Man can be spotted when the player enters Joe's boxcar home. Right as you enter, look at the TV that the Vortigaunt is watching (and powering). The G-Man is on it.

Trivia
On the website Planet Half-Life, the story Parabellum is told from Joe's point of view, although in it he is named Jack Harper, and his Vortigaunt friend is named Oscar.