Dr. Gordon Freeman
You are Gordon Freeman. Aged 27 and an MIT graduate (Ph.D. in Theoretical Physics), you begin work as a research associate in the Anomalous Materials Laboratory of the Black Mesa Research Facility, all thanks to an unidentified administrative sponsor.
Appearances In Half-Life Canon
Being the player, Gordon Freeman is present at almost all points of the Half-Life saga. In the original game, you start on an electric tram within the Black Mesa Research Facility on your way to work one morning. After arriving late at the Sector C Test Labs & Control Facilities you head down to the Test Chamber and accidentally cause the Resonance Cascade. From here you travel through Black Mesa, towards the surface to get help, but have to turn back when you are confronted by hostile military forces. You then head into the Lambda Complex and finally to the borderworld Xen, where you approach and defeat Nihilanth. You are then conveniently extracted by the mysterious G-Man and given a choice to either work for him to whatever end or face certain death in Xen.
Although this is the last time the player takes control of Gordon until Half-Life 2, he is visible from afar in certain areas of Opposing Force, accomplishing tasks the player would have done over the course of the original game.
Half-Life 2 assumes that you, Gordon Freeman, chose life over death and were entered into stasis by the G-Man. Now that you are needed again however, you are given a short speech by your new employer and dropped into an arriving passenger train at City 17. Over the course of the game you travel through the city, its canal system, the nearby town of Ravenholm, the coastline, and the Combine prison, Nova Prospekt. After escaping the prison through a portal with Alyx, you return to the city a week later to find your blow against the Combine at Nova Prospekt has sparked a revolution. Teaming up with the rebellion in the city and Barney, you lead an assault through the city streets and eventually enter the Citadel alone after passing through the Nexus Building. After helping to free Alyx and Eli, you destroy the device by which Dr. Breen had hoped to escape and with it the entire Citadel. As soon as this happens you are again swept away from the Citadel by the G-Man, and once more entered into stasis.
Gordon Freeman is set to next appear in Half-Life 2: Episode 1, where your journey will continue through the events immediately following the destruction of the Citadel.
Theories
There are a number of theories about the future and past of Gordon Freeman, given the key role he plays in most of the important events of the Half-Life universe.
For example, at the start of the game the onscreen text reads "Sponsor: Classified". It is unclear why this is classified or whether it is even significant, but many believe that your sponsor is the enigmatic G-Man, as part of his attempts to shepherd you into, and perhaps through, the events of the Black Mesa Research Facility.
Equally, there are many theories surrounding Gordon's relationship with the equally mysterious Dr. Breen. It was he who chose the samples that were used in the Test Chamber the day of the Resonance Cascade, but it is unclear whether he knew what would follow or whether he specifically wanted Gordon to be involved. What is clear, however, is that at the end of Half-Life 2, Breen somehow knew about your business relationship with the G-Man, and threatened to tell Eli Vance, Alyx Vance and Judith Mossman about it.
It remains to be seen exactly why Gordon is such an important part of the events in the Half-Life universe.