Quiller is an agent for The Bureau.
The first line of action known about Quiller was the time spent during the latter months of the war behind enemy lines, masquerading as a German soldier assigned to several concentration camps, working in reality to free as many of the condemned Jews as he could. When the war was over and his activities made known, he was offered medals from several countries for his bravery but rejected them all out of frustration for not having saved more.
Shortly thereafter, Quiller was recruited by the Bureau, an organization that 'didn't exist' because, as is explained many times, if it did exist, it couldn't do the things it was needed to do.
In the Bureau, the people also didn't exist. Their real names are never mentioned and are only recorded in the files of the upper echelon. No facts about the personal life of the agents became known. Quiller was only the cover name given to this agent. Since it was only a cover, it didn't matter whether the name was a first or a last.
In the first novel, Quiller's adversary, in recounting what was known about the British agent, stated, 'You are a known authority on memory, sleep-mechanism, the personality patterns of suicide, fast-driving techniques and ballistics.' These are just a few of the areas of expertise that allows him to survive all the years in the Bureau.
Quiller-9 is the full designation for this agent. The 9 suffix is assigned to the few executives who have been designated 'reliable under torture' and therefore not required to carry the cyanide pill that would ensure security. Quiller laments that, unfortunately, the only way to get such a label is to have actually proven the point, not a pleasant way of earning it.
Quiller is an anachronism is the spy business, not only in the Bureau for whom he works but also in the entire field of spy-hero fiction. He doesn't like guns. It isn't that he is bad with them for he has quite a good aim and a superb knowledge of the attributes of most weapons. It is that he feels carrying a gun would make him prone to rely on it and use it when thinking would be a better choice of action. This lack of firepower has gotten him chided on more than one occasion but it has also bewildered adversaries who cannot believe that he would not have a gun.
The lack of a firearm, however, does not indicate passivity. Quiller is highly skilled in the martial arts and the hand-to-hand combats that are described in his adventures are probably the best written and most believable to be found in any series. He is no superman but he is good.