Sōzō's visage, despite his wartime experiences, displays a soft essence of innocence and peace. He is a handsome young man with shiny black hair that is well-groomed in the back, but flows in sweeping bangs which fall over the right side of his face, obscuring his right eye. Even in death, his face displayed no pain. Sagara's attire during his depiction in the series consists of a standard Sekihō army officer's uniform (including the red bandanna identical to Sanosuke's)and he is constantly seen carrying himself with grace and pride.
Sōzō had a caring and ambitious personality, wanting to help create a Japan where people live under freedom and equality, much like Himura Kenshin. A natural leader and mentor, he was fond of children and a champion of the common man. He did his utmost to teach these principles to those under his lead and it is because of these ideals that Sanosuke befriends Kenshin, despite him being a former Ishin-Shishi.
Not much is known about his abilities, however, considering that he was the leader of the Sekihō Army, it is likely that he was an extraordinary combatant and a capable commander.
Born Kojima Shirō, the captain was not born into a samurai lineage but rather became the scion of the wealthy Sagara family. Perhaps as a result, he came to dislike the haughtiness of samurai and the great difference between classes during the Tokugawa dynasty's rule. When the Bakumatsu began, Sagara left his wife and family to join the Sekihō Army--a militia of farmers and merchants fighting on the side of the Ishin Shishi. He was promoted to Captain of the first squad and took on the responsibilities of leading the makeshift force and spreading news of the Meiji Government's promise to halve taxes. Sōzō took great pride in his role during the bakumatsu and used his influence for the sake of the people, eventually taking two poor boys, Sanosuke and Katsuhiro, under his wing as wards of the Sekihō Army and teaching them his ideals and principles. However, when the new government reneged on its promises of tax reform, it marked the Sekihō as lying frauds and attempted to purge them from its ranks. Seeking compromise, Sagara reported willingly to his Ishin Shishi commanders in Shimosuwa, but rather than explaining, the government had him and his comrades executed by decapitation and put his head on public display. Though the truth of Sagara Sōzō's heroism and sacrifice was buried by the Meiji Government and unknown to the general public, the Captain's noble life and ignoble death would serve to shape the worldviews of Sanosuke (who took on the surname of Sagara to honor his hero) and Katsuhiro throughout their adult lives.
In Act 73, while Sanosuke is training in the forest near Shimosuwa, Captain Sagara appears before him as a spirit, asking his former ward to consider his life more carefully.