Sam Baxter, aka Steelhead Sam, is an agent with British Intelligence.
When we first meet him, he is very happily a sergeant in the vaunted Parachute Regiment of the British Army having made it his life's career and expecting to stay there for many more years as he is (likely) still in his late 20s. Then in that first recorded adventure, we witness his heroic and determined efforts to stop a terrorist attack against an incoming foreign ambassador. His quick action prevented the man's death but an RPG from the assaulters as they fled severely injured Baxter.
"The bones in your skull and face were so badly damaged that we had to completely rebuild you. The mask is made of a new type of titanium ... far lighter and stronger than any other metal." is what the doctor tells him after the operation to repair him. A look in a mirror lets Baxter know "you've left me looking like a blinking robot!" and he is correct with most of his cranium now a hairless shiny thin plate.
His CO tells him that he is on indefinite leave; i.e., they had no idea what to do with him since he was otherwise in remarkable shape. He has ideas, though, when he is told he is out of picture when it comes to tracking down the terrorists. He had gotten a good look at the leader and with a lot of time on his hand and one major burning anger inside him, he starts his own investigation. He successfully bullies his way to finding the group and smashing their cell, literally.
In doing so a couple of major things take place. He finds that though his appearance is bizarre and off-putting, he is able to shrug off a bullet to the head as well as use his noggin to smash through a door, giving him an impressive edge. And he finds that the cell was under surveillance by the SIS and though they are miffed he stepped in when he did, they saw a major use for him.
As the head of Special Operations for MI6, Colonel Trueman, puts it, with the use of face masks "made of a special rubber to give them a lifelike appearance", Baxter could be quite an operative for them, especially since there were "a dozen different ones" for him to use as needed.
That is when the single Baxter, self-named 'Steelhead', finds a new career.