Herbie Kruger is an agent with British Intelligence.
Born in Germany before WWII, Kruger was a young teenaged war orphan at the end of the conflict, struggling to survive. American soldiers started using him to infiltrate different refugee camps and other gatherings, looking for information to separate the good guys from the Nazis. After a couple of years, they "passed" him on to the British Intelligence operative for much the same work.
As he reached adulthood, now with an excellent grasp of English as well as his native German, and now also well versed in spy craft, Kruger was offered a position in British Intelligence, along with citizenship. With nothing to lose and a growing affinity to England, he accepted.
Thus began two decades of solid intelligence work as Kruger made use of all his knowledge and contacts to create an impressive spy network in East Germany that had reaches even into the Soviet Union.
Nothing lasts forever and eventually someone talked. The network started to get picked up. Kruger worked tirelessly to save as many as he could and he was successful enough to get a healthy respect from his superiors and peers.
As the series begins, though, the 70's are quickly winding down and a great operative and spy master from the 50's and 60's is considered archaic to those coming up who would function in the 80's. Technology has changed so much and the atmosphere in the world was so different, it is hard for Kruger to have any relevance.
This large, bumbling-looking, aged agent is still at the top of his game but few know it. And even fewer realize that the bumbling movements, the shy demeanor, the almost stupid deportment are all tricks of the trade to make everyone discount him. Unfortunately, now his deceptive abilities have worked too well.
Note that the author, Mr. Gardner, also wrote a three-book series called the Secret Trilogy. In the third book, Kruger makes his entrance as a young agent. His role is fairly small but important. It is more important in the understanding of the character in this series.