Paul Kilgerrin is a private investigator who is also an agent with Military Intelligence.
Possibly based in California, though he yearns for the big city of New York on more than one occasion, and having been a private eye for nearly two decades, this hard-boiled, no-nonsense detective has little in the way of sense of humor and often even less social skills but when it comes to going undercover and getting the job done, he is superb. He has never been married and, although he has a fond eye for the ladies, he seldom gets close to any, with one exception. He does not seem bothered by his lack of companionship and would look at you strangely if you mentioned it.
While his age is not mentioned, his history, mentioned in various places, would put him in his mid to late 40s. He served with distinction and valor during the First World War. Assuming him to be in his early 20s in 1917, the recorded adventures take place a quarter century later which would make him around 45. This is somewhat confirmed by the fact that he thinks on numerous occassions how many nasty situations he has managed to walk from and he speaks with voice of someone who has been there and done that and is a bit weary from it all.
Kilgerrin gets shot and knifed and beaten a fair amount in the series and he does not bounce back easily from the injuries, making him a welcome change from the scores of near-supermen that sometimes grace the pages of spy fiction. He had suffered a very bad wound during WWI and took a long time recovering. As the series opens, he had just been involved in another gun battle and earned yet another injury. Though he had healed from both of those, his ability to shrug off new infirmaries is greatly tasked, giving an air of realism to the traumas.
Kilgerrin did not go looking for work with Military Intelligence as a private consultant once the Second World War began. He tried several times to reactivate his commission but the injuries he had suffered at the end of the first kept the military from accepting him. When the head of the department for which he would eventually do so much of his clandestine work, Colonel Mathewson, ordered him to come for a visit, there was no love lost between the two. Mathewson did not private investigators and did not like the reputation Kilgerrin had earned as one - being a man who followed his own direction and often ignored the law if it was necessary. Kilgerrin on his part was miffed that Mathewson and his group had chosen to reject his offer of help only to then ask for it covertly. The first mission was filled with considerable tension as it took a while for each to warm to the other but the thaw did happen and they soon became extremely close to other.
In many of the adventures helping Kilgerrin is Gerry Cordent, a female test pilot related by marriage to Mathewson and recently widowed when her husband died in a test flight. A lovely woman with beautiful copper-colored hair, Cordent is as brave as they come and willing to go more than the extra mile to help Kilgerrin and Mathewson. The two do not become romantically involved but rather quickly grow to enjoy a most rare relationship in spy fiction - a mutual respect and deep friendship.
Note: One fact that makes the Kilgerrin series so interesting is that he is most definitely a hard-boild detective with his own set of rules, willing to kill the bad guy without hestitation and suffering a great many physical attacks as he charges into deadly situation. He is truly a man's man when it comes to the rough stuff. The interesting fact is that Charles L. Leonard, the creator and writer, was really Mary Violet Heberden, a woman in her mid 30s when she started the series. Knowing it would not be well received if her gender was revealed, she kept it quiet for many years.