Dr. Gordon Freeman
The man. The myth. The glasses. Gordon Freeman is the famed MIT graduate (PhD, Theoretical Physics) that stars in the heroic role of Half-Life and Half-Life 2. At the age of 27, he gets a job working at Black Mesa Research Facility as a research associate in the Anomalous Materials Laboratory, thanks to an unidentified administrative sponsor.
Being the player, Gordon Freeman is everywhere. In Half-Life, he starts in an electric train system within Black Mesa. He travels through Black Mesa, into the Lambda Complex, and finally to the borderworld Xen, where he approaches and defeats Nihilanth. He is then conveniently extracted by the mysterious G-Man.
In Half-Life 2, Gordon returns, arriving on a regular passenger train into City 17. He travels through the city, its canal system, Ravenholm, the coastline, and Nova Prospekt. He returns to the town a week later, and leads an assault on the Citadel. Yet again, he is conveniently removed from the Citadel by the G-Man upon stopping Dr. Breen's escape.
He is known to appear in Aftermath, but what he does and what happens to him is still unknown.