Acer Sansom is a British soldier and part-time agent of British Intelligence.
When we first meet him, it is in a hospital in England where the answer to the question whether Sansom was dead was, "Yes ... and no ... and then again, probably". In explanation for this ambiguity, the police detective tells a visiting military officer that the "Public Records Office says he is [dead]" but the still barely-breathing form in the bed clearly indicated he was still among the living, though the attending doctor did not hold much hope for he staying that way.
At the time that Sansom was declared dead six months before his reappearance. The cause of that declaration was the belief that he had died along with his wife and baby daughter, as well as the other passengers and crew of The Rendezvous when that cruise liner disappeared without trace in the Pacific. He had, however, survived but was marooned on an island for a year. Returning to England, Sansom's only desire was to learn who had been behind the massacre on that vessel. That investigation was what led to his being on the hospital bed ready to die a second time, this one being for real. Of course the doctors had no way of estimating the determination for revenge and justice that filled Sansom.
Sansom is described as being "exceptionally tanned, which was accentuated by his shoulder-length, sun-bleached-blond hair. His face and torso showed him to be lean, bordering on gaunt, although his arms and chest indicated strength. A good-sized tattoo covered much of his right bicep. He looked as though he would be very fit..."
As mentioned above, Sansom is a soldier and he was happy being one. His secondary role with British Intelligence comes from his search for justice for his family and having to go up against some very powerful people, some of whom were also being watched by the government.