Kaio is the main antagonist of the Shimabara Arc.
Kaio is a man who self-raised himself in China after his mother, a Japanese kakure kirishitan refugee, escaped from prosecution many decades ago. When his mother died due to depression and illness from grief of exile, he from then on harbored extreme hatred for his once homeland. He meets Shōgo there and becomes his right hand man and priest.
Though loyal and supportive, Kaio reveals himself to be treacherous and manipulative. He unveils to ultimately use Shōgo to push his secret plan of using him as a martyr to Sanosuke, Saya, and Shozo. In doing so, he plans to garner the unanimous Christian fervor of Western powers to support them, and then unite a head on front to attack Japan to overthrow the Meiji government, before rendering Japan a colony state to fully Christianize the nation by force. He is killed by Shōzo Lorenzo when the latter throws bombs (made by Tsukioka Tsunan, given to him by Sanosuke) into his boat as he tries to escape.