Onmitsu Kenshi

The adventures of a roving samurai detective/spy, Akikusa Shintaro whose real identity is Matsudaira Nobuchiyo - the older half-brother of Tokugawa Ienari, the 11th Tokugawa shogun, who is still a minor. Because he is the son of a concubine, Nobuchiyo has no claim to power, so he assumes the guise of a wandering swordsman named Shintaro to seek out and eliminate plots by rival feudal lords and thus protect his younger brother.