Nanbu, named for his father, was the son of Nanbu, a simple budoka of Otosan Uchi, who in the 2nd century saved the life of Soshi Saibankan, becoming the personal guard of the famous Emerald Magistrate. He was raised in Saibankan's service, and after his father died on duty, he continued to serve Saibankan, and after him to his daughter Soshi Sayoko. Nanbu nearly died uncovering
a rival's plot to poison her, and she promised him that so long as his sons carried his name, that they would serve the Soshi. A century later, their descendants eventually would be granted by the Soshi as the Nanbuvassal family.