Dr. Gordon Freeman
Gordon Freeman is the protagonist of the Half-Life series. Originally employed by the Black Mesa Research Facility, Freeman is one of the main survivors of the Resonance Cascade that causes the events of the series to unfold.
The player almost never sees Freeman, which is an intentional stylistic choice on Valve's part to enhance the immersive gameplay of the series. For all intents and purposes, the player IS Freeman, almost never losing control of him and allowing the player to project themselves onto him. Instead of the storyline being told through cutscenes, it is instead told through the environment and through scripted sequences involving the NPCs around the player. As such, Freeman's profile is left intentionally vague, to the point where a Freeman model isn't even included with the game.
Appearances In Half-Life Canon
Freeman is a 27 year old Ph.D in theoretical physics. His task on the day of the resonance cascade is simple: place the sample into the testing equipment. This opens up a dimensional rift which causes aliens to be teleported en masse onto earth. Throughout the original Half-Life, Freeman battles aliens and the military on his way to the Lambda Complex in order to close the rift and prevent more aliens from getting through. After this goal is accomplished, he is put in stasis by the mysterious G-Man, awaiting further instruction.
Freeman is woken up at the start of the second game and is put on a train heading straight towards the Combine empire in City 17. Freeman reconciles with Barney and Dr. Kleiner from Black Mesa and is sent to Black Mesa East to help out with further research into defeating the Combine. After arriving, he gets derailed off this path and is sent into the zombie-infested Ravenholm.
We soon learn that the Combine have taken Eli Vance from Black Mesa East and are taking him to the Citadel. Gordon heads through the coast and completely destroys Nova Prospekt, which causes hundreds of rebels to join forces and further the Resistance movement. Through his time battling through Black Mesa and Nova Prospekt, Gordon becomes a legendary hero in the movement.
After barreling through a war-torn City 17, Gordon finally breaches the Citadel and fights his way through hundreds of Combine soldiers with Alyx on his way to stabilizing the core of the facility. City 17 completely collapses and Gordon and Alyx now head towards White Forest to deliver important packets of information to Eli and Dr. Magnusson.
Now at White Forest, Gordon and Alyx have to help the scientists prepare a rocket with which to launch a satellite array to close the Combine superportal. It is revealed in Episode 2 that Gordon is no longer under control of the G-Man, which was a key plot detail driving the previous games.
Behind the Scenes
As noted, the player never sees Freeman. The only time this happens is in the Gearbox expansions of Opposing Force and Blue Shift, as well as in Half-Life 1 multiplayer as a model. In Half-Life 2, Gordon is never seen and a generic version of his HL1 model with dev textures is used instead. (Please note that although an official Source model of Freeman exists, it has only begun circulating in late 2012.)