Shōzo is an affiliate of Amakusa Shōgo and Amakusa Sayo. As a child he was harrassed and bullied (presumably due to his Christian faith) until Shōgo saved him from his tormentors one day. Since then, Shōgo and Sayo looked after him and he remained extremely loyal to them.
Though initially enemies, he befriends Sagara Sanosuke after the two of them are determined to crush a plan by Kaioh, a traitor to the Shimabara Christians who intends for Shōgo to be sacriced, so that he can make Japan a colony for foreign powers, which he rules over. After Shōzo loses to Kaioh in a fight (being severely injured in order to save Sayo and Sano earlier on), he faces no choice but to break Sayo's teachings by taking Kaioh's life in order to thwart his plans, using Tsukioka Katsuhiro's bombs given to him by Sano. After Sayo dies, Shōzo is grief-striken and later departs for Holland along with Shōgo and the other surviving Shimabara Christians.