Based on the actual captain, Sagara Sōzō (1839 - 1868) was the leader of the Sekihōtai's First Troop. Sanosuke looked up to him greatly; he was, to Sanosuke, the ideal role model. He was killed when the Meiji government purged the Sekihōtai, leading to Sanosuke's embitterment towards it. His head was placed on a platform and in a cage for all to see, it haunts Sano still to this very day. Sometime after his death, Sano took his name and became Sagara Sanosuke.

(Source: Wikipedia)