The Oniwabanshuu were a group of ninjas/onmitsus established by the 8th Tokugawa shogun, Tokugawa Yoshimune (1684-1751), who is considered to be one of Japan's greatest rulers. His far-reaching reforms totally reshaped the central administration structure and temporarily halted the decline of the shogunate. He was originally the head of Kii, one of the three hereditary Japanese feudal fiefs ruled by descendants of the original Tokugawa ruler not in the main line of succession to the Shogunate. (The other two fiefs/hans were Owari and Mito - together, the 3 hans were known as the 3 Houses, or Sanke.) A lack of sons in the main branch of the family, though, resulted in Yoshimune's succession to the position of Shogun in 1716.
Yoshimune established the Oniwabanshuu for security, information gathering and spying activities. Originally, members of the Oniwabanshuu were chosen from the Kii clan, and was passed down in the families - in other words, the posts of the Oniwabanshuu were hereditary. They acted as security guards in the Edo castle, besides spying on other clans of their policies and actions. It was also believed that in Bakumatsu times, Oniwabanshuu members were placed among the missions sent to America.