Freya Willims and Stone Smith are agents of CURE.
More accurately, they are novice agents of a brand new branch of that highly secret government agency that no one but the President knows exists. Its director, Harold Smith (no relation to Stone Smith), decided that a good deal more attention was needed on border infiltration and he created this offshoot to concentrate on it. The man in charge is assigned these two rookie agents to be his entire force.
Williams and Smith are half-siblings, sharing a common father but raised separately and unaware of each other for much of their lives. They knew only slightly more about their absent father growing up just as he did not know of them for quite a while. When they learned of him, and he of them, their lives changed dramatically for he was Remo Williams, aka , the current Master of Sinanju, and one of the deadliest assassins on the planet.
Remo Williams has been an agent of CURE, and a disciple of Sinanju, the sun-source of all martial arts, for several decades. Trained by the then-reigning Master, for tradition allows only one Master at a time, Chiun, he has become one of the greatest Masters in the 5,000 year history of the Korean House of assassins. Since taking the title as Chiun reluctantly assumed the role of Master Emeritus, he has worked for CURE fighting the enemies of the U.S. while earning the money needed to keep the North Korean village of Sinanju alive. Along the way in his many missions, Remo had an occasional dalliance or two. A couple of them resulted in children he only became aware of later.
The older of the two is Winston "Stone" Smith, the product of Remo and a beautiful nurse that he met when he was first brought to the headquarters of CURE. The CURE Director knew that having a family would distract Remo from the jobs he had to do so the information was kept from him. The child grew to be a man and that man became a Navy SEAL. Despite the military training, however, there were time when Smith would act out and one such event forced his being sent, for his own good, to live with his paternal grandfather, Remo's father, William Rome.
Freya Williams is a good ten years younger than her brother. Fifteen as the series begins, she is the child of Remo and the impressive Jilda of Lakluun. Jilda was the best warrior that a reclusive Viking civilization on a remote island in the Faeroes of the Norwegian Sea, and as such she was sent to combat Remo during a ritual as he neared reaching Master status. Instead of killing him, or being slain by him, she fell in love. When she learned she would bear his child, she also knew that child would always be a target so she kept the truth from him until the child was 5. Even then, when Freya and Remo finally met, it was agreed that the girl would stay with her mother. Only when the mother was killed years later that a newly orphaned Freya was sent to be with Remo's father.
William Rome, the grandfather suddenly entrusted with the two wards, was himself a descendant of a Sinanju offshoot, known as the Sun On Joe tribe of Arizona (long story). His knowledge of Sinanju fighting is tremendous, though nowhere approaching his son's. It is good enough, however, to make the two grandchildren, now his students, extremely dangerous to upset.