He was a low-ranking samurai who was adopted by the Katsu family in order to marry the only Katsu daughter, Nobuko. Kokichi's father, Otani Heizo, was a minor official in the shogunate. His half brother, Otani Hikoshiro, was twenty-five years older than Kokichi. After their father's retirement as family head, Hikoshiro became responsible for all the family; he was a noted calligrapher and Confucian scholar and was twice distinguished as district administrator within the shogun's domain. By contrast, Katsu Kokichi led a life of idleness, never achieving an official post and supplementing his small (41 koku) income by dealing in swords, among other things. The other things, contrary to samurai-class ideals, included acting as a security guard and lending money at high interest. When Kokichi's son Rintaro (later to become the famous naval commander Katsu Kaishu) was fifteen, Kokichi retired as family head, passing on that duty to young Rintaro. During his last days, Kokichi wrote an autobiography (one of the few surviving from pre-Meiji Japan) titled Musui Dokugen (Musui's Story), narrating his life and adventures.