Seta Soujirou is Shishiou Makoto's right hand man, a teenage assassin trained by Shishio himself. Soujiro is known for his lack of kenki or, simply, emotion. He possesses no anger and cites no fear in his enemies, other than with his sword.
Soujirou is often seen smiling and has polite manners, always referring to others with Japanese suffixes. He is sarcastic, hiding his true emotions behind his smiling face. He however does have a special relationship with Yumi, she being a sister figure towards him. Soujirou usually dresses up in a blue kimono with white handguards
Soujiro's constantly smiling face and polite manners stem from his tragic past. Born in Kanagawa Prefecture during the first year of Bunkyuu (1861), Soujiro is an illegitimate child. Sōujiro's father left him in the care of his family, who badly mistreat him. He is assigned chores such as carrying rice to and from the family warehouse, and whenever he faltered, he was beaten by his family. Soujiro built a defense against their lashings; he put on a smiling face, and endured the torture until they got tired of beating him and left him alone.
One night, he meets Shishio, who was on the run from the police. Soujiro witnesses Shishio kill some police officers and Shishio is about to kill him for it until he notices Soujiro smiling and therefore spares him in exchange for food and bandages. Shishio tells Soujiro that only the strong may live and that his predicament is his own fault for being weak. This new mentality, and a sword given to him by Shishio, determine his defence when his family finds him housing Shishio. Sojiro quickly finds himself cornered by them, attempting to murder him and blame his death on Shishio, and lashes out, killing every one of them at the age of eight.
Soujiro''s apparent lack of emotion, his simple, insensitive mentality that the strong will live and the weak shall die, and his skills make him a formidable foe for Kenshin. As a master swordsman of his own style, Soujirou's abilities have earned him the nickname of "Heaven's Sword" (天剣, Tenken?, Ten meaning Heaven and Ken meaning Sword, "Tenken" means swordsmanship of natural endowments). His shukuchi utilizes such powerful speed that, to an observer, it appears that the distance between the two fighters has shrunk (hence the name shukuchi, which means "reduced earth").
During his fight with Kenshin, Soujiro continually states he's only moving "three steps below shukuchi" or "two steps below shukuchi." While moving at the speed his shukuchi utilizes, the only thing that can be seen is Soujiro's footsteps ripping through the tatami mats. During "one step below shukuchi", he is able to attack from an omni-directional angle, utilizing not only horizontal but vertical space While performing true shukuchi, he completely disappears and cannot be seen, even by Kenshin. Soujirou can dodge the Kuzu-ryūsen (九頭龍閃?, lit. "Nine-headed Dragon Flash"), a move which apparently cannot be evaded even by Kenshin himself. Soujirou is also the first one ever to slash Kenshin in the back in the series.
Soujiro is also well versed in the Battōjutsu style like Kenshin, as is evident when he decides to use a stance similar to Kenshin's and when he destroys Kenshin's first sakabatō. Combining the true speed of shukuchi with the Battōjutsu, he has created his only named technique, Shuntensatsu (瞬天殺?, lit. "Instant Heaven Kill"), meaning "immediate killing by heaven's sword". It is named this because the speed behind the attack is so swift that the opponent dies instantly before falling to the floor.
(Source: Wikipedia)
