[3] Nobuhiro Watsuki has stated that the character design of Sōjirō is loosely based on Okita Sōji, the captain of the first Squad of the Shinsengumi.

Okita died when he was young, but Watsuki decided not to kill Sōjirō in the story in order to atone for the murders he committed, but instead start living on his own instead of having someone make his decisions for him.

Sōjirō is known for his lack of kenki (offensive aura) in a fight, his apparent emotionless state reflected also in his sarcasm and polite manners with his smile hiding his true feelings.

Born in Kanagawa Prefecture in September of the first year of Bunkyū (1861), Sōjirō is an illegitimate child who was badly mistreated by his father's family to the point of being beaten for faltering in his tasks.

One night, at age six, Sōjirō witnessed the recently scarred Shishio Makoto killing two police officers and was spared due to the criminal seeing the boy's smile.

When Kenshin finally arrives at the Room of Reduced Space in Shishio's lair, he finds Sōjirō waiting for him and at first has the upper hand due to his superior speed.

But when the confrontation forces Sōjirō to question Shishio's principles, his true emotions surface with allowing Kenshin now able to read his moves and defeat him.

Five years later, he makes his way to Hokkaido where he is reunited with fellow Jupongatana member Yūkyūzan Anji, who was being escorted to Hakodate by Sugimura Yoshie (Nagakura Shinpachi), and joins them.

Sōjirō's apparent lack of emotion, his simple, cold-blooded mentality that the strong will live and the weak shall die (弱肉強食, jakuniku kyōshoku, an idiom for "survival of the fittest" or "law of the jungle"), and his skills make him a formidable foe for Kenshin.

It is named this because the speed behind the attack is so swift that the opponent not only dies before falling to the floor, but there isn't even time to experience pain.

[11] The inclusion of Seta Sojiro in The Final was praised for Ryunosuke Kamiki's performance as the character became an aid to Kenshin and wants to see his resolution to his life as a pacifist while also enjoying the bigger focus on Misao and Saito.