K-51 is an agent with the American Secret Service.
Z-19 is an agent with the American Secret Service.
The series is definitely that of K-51 for his is the codename decorating each adventure and he is the focus of each one. In most, though not all, however, he has the partnership of fellow agent Z-19. For that reason, she gets nearly equal billing.
The term "Secret Service" may or may not be a catch-all term for American Intelligence. Regardless, K-51 is an internationally traveled and recognized agent of considerable repute. Sometimes too easily spotted for who he is, K-51 is known to be a dangerous adversary so his enemies are often on their guard against him. Being so apparently well known, however, has its advantages because bad guys seem to relish explaining their plots to him.
The missions that K-51 get sent on are by no means easy or light-hearted. In the third recorded adventure he heads to China where he meets a diabolical would-be world ruler named Lin-Sun and her evil machine to kill from a distance. The first victims are residents of a convent and school in a nearby village. Innocent children and nuns are shown "falling to Earth dead". Many another innocent will die at the hands of bad guys who are hunted by K-51 showing how vital his efforts are.
K-51 is a blond-haired good looking man with a pencil-thin mustache (at first but a lot bushier as time goes by) and a propensity for sudden action. He loves using his fists and uses them freely. The same holds for his gun hand which dispatches a fair number of adversaries. He is not always victorious at first, though, and he must surely have a cast-iron skull considering the number of blows it receives on his every mission.
While we never learn his real name, we can see over the many adventures he has that his lack of a known name is not a hindrance since apparently everybody knows him by his codename. We have no information as to his background other than he is likely well-to-do because he has a personal valet named Wong, seen in a few occassions. In one adventure half-way through his recorded career he is visited by a scientist with some incredible powders he has developed which give K-51 super-strength (and one that lets him shrink to a few inches) but he only used his "vitamins" a couple of times before he apparently ran out and they were never mentioned again.
Early in his recorded adventures he comes upon a woman he suspects is an agent for a foreign power. He is partially correct in that she is an agent but it is for his own organization. She is named Claire (no last name given) and she has the codename of Z-19. They save each other and complete the mission, getting friendly along the way. By the end of the next assignment which they happen to share, they become engaged. From then on, though he often works alone, he is joined on many an occasion by his lovely bride-to-be and she shows over again how capable she can be as an agent.
On at least one occasion, K-51 is seen getting his marching orders at the F.B.I. headquarters and he is once referred to as a G-Man but I found no evidence he was in fact a Fibbie, especially since near the end of his tales, K-51 is promoted to Chief of the Espionage Bureau of the National Defense Committee. That does not stop him from getting personally involved in cases.