Seta Sōjirō was the fastest member of the Juppongatana. He was the right-hand man of Shishio Makoto and the most skilled swordsman among his crew. He was usually seen by Shishio's side. When not with Shishio, he was off on missions such as assassinations or gathering the other Juppongatana. His nickname was "Sōjirō the Tenken". He believes in Shishio's philosophy that only the fittest survive. "If you're strong you live, if you're weak you die". His favorite sword was the Kikuichimonji-norimune, which was cracked by Kenshin's sakabato during a contest of Battōjutsu. He is the one responsible for the assassination of Okubo Toshimichi.
Sōjirō is normally seen smiling, with seemingly no emotion. In actuality, he subconsciously suppresses his emotions, making him difficult to read in combat, as stated by Kenshin. This façade gradually erodes when he realizes how strong of an opponent Kenshin is. He also adheres to the philosophy of "the survival of the fittest" and believes it to be an absolute truth. His dedication to this belief, however, leaves him emotionally vulnerable and naive when another's differing ideology conflicts with his own. When his true emotions surface, he appears to be unnerved, infuriated, and almost insane. This is a direct result of spending over a decade suppressing his feelings due to the trauma and abuse he suffered as a child.
As a child, he was a kind calm patient child who smiled often to avoid the abuse his father's family often caused due to him being a product of infidelity from his father and a prostitute, something that brought great shame upon his father's family. He would obediently follow their orders, no matter how ridiculous and labor-intensive, and endured much of their abuse and bullying, which was unjustified and cruel. He only broke this smile upon first meeting Shishio, whom he was scared of. However, he quickly grew attached to him and Shishio taught him that the strong live and the weak die. When his family threatened to kill him to get the reward money and the fact they wanted to get rid of him for good, a part of him snapped and he had no choice but to kill them before they killed him, when he killed his older half brother, the other members of family were horrified and he ended up slaughtering them all. While on the inside he was crying because of what his family wanted to do to him and how they hated him for something that wasn't his fault, he outwardly smiled to hide the pain and grief because despite their abuse and clear lack of love for him, he still thought of them as his family. This abuse and trauma led him to become emotionless and always hold a smile, though he was broken on the inside.
Eventually, after Kenshin fought him it began to clash with his ideas of the weak and strong that Shishio implanted in him. This is because to him Kenshin was weak since he couldn't kill his opponents and often defended those weaker than him. In addition, this caused him to doubt the very philosophy that allowed him to live for so long and keep hold of what little sanity he had. A part of him also wondered why someone like Kenshi didn't save him and teach him better. Despite having had to kill multiple people during is life on orders of Shishio, it is hinted a part of him never wanted to kill people and he had only done so to keep with the philosophy of strong and weak. Eventually, Kenshin teaches him that he needs to develop his own life philosophy through his own experiences rather than blindly follow Shishio. After the Kyoto Arc, he appears to have calmed down somewhat and gone back to his happy-go-lucky self while going on a ten-year journey to find his own philosophy of life.
During his time under Shishio's tutelage and service, Sōjirō seems to regard him as a combination of master, god, and foster father.
Sōjirō was an illegitimate son of a rice business man and was left in the care of his much older half-brother. It's unknown what happened to his mother. As a child, Sōjirō was abused and beaten by his family as he was a constant reminder of his father's infidelity and the shame he brought upon his family. Sōjirō's father was never punished or shamed for it personally by his wife or adult children and Sōjirō was an easy target for their anger since he was a defenseless child, and he would smile instead of cry to make them stop temporarily. One night, he ran into Shishio Makoto, who sliced a government official in half. Instead of killing Sōjirō, Shishio took shelter in a shed and Sōjirō would bring him food and bandages. He gave Sōjirō a wakizashi to protect himself by killing his family, but he thought being killed by a sword would hurt too much so he hid it under the porch. When his "family" found out Sōjirō was harboring a wanted criminal, they planned to kill the boy and turn Shishio in, causing Sōjirō to snap and he had no choice but to kill his abusers before they could kill him. Sōjirō ended up slaughtering the five adults with great speed and strength, the result of believing what Shishio told him; "The strong live and the weak die". Afterwards when he stood in the rain, he seemed to be smiling about it, but he was actually crying and was angry with his family. He decided to go with Shishio.
Kyoto Arc Edit
In his first appearance, Sōjirō killed Okubo Toshimichi under Shishio's order, his abilities made his presence unnoted. After Kenshin defeated Senkaku, Sōjirō engages in a fast duel with him, resulting in a tie. Then, Shishio ordered Sōjirō bring to him The Ten Swords from the East side of Japan.
Sōjirō was the last member of Juppontagana that Kenshin must defeat before facing Shishio. He used Shukuchi's less potent forms to move fast enough to hit Kenshin, both managing to dodge the Kuzuryusen and slash him in the back using merely the second step behind the true Shukuchi. As the fight dragged on however, Kenshin made Sōjirō question the reasons that made him fight alongside Shishio, and this made Sōjirō release his true emotions, allowing Kenshin to predict his moves and discover his advantages. Sōjirō got mad and rendered his emotional blankness null; the battle reaches the climax when Sōjirō challenges Kenshin to do a last technique to put an end in the duel, Sōjirō used Shuntensatsu, the combination of the full Shukuchi and his Battojutsu against Kenshin's Amakakeru Ryū no Hirameki. The two attacks were comparable in overall speed, but Kenshin's ōgi was far stronger and instantly broke Sōjirō's sword and left him battered. Sōjirō declared that his loss showed he had been wrong, but Kenshin instead said that the victor isn't right by default and he must discover his own truth before leaving the room to face Shishio.
Sōjirō discovered the key to Amakakeru Ryū no Hirameki and informs Yumi to tell Shishio. Sōjirō asked for Yumi to return the Wakizashi given to him by Shishio. Then, he leaves the Juppongatana and became a traveler once Shishio died, intending to travel the next 10 years to discover his own true self while being on the run as a fugitive. Cho stated his confidence that Sōjirō would never be caught.
- Shukuchi (縮地, Reduced Earth) Sōjirō moves at beyond god-like speed, only showing his footsteps crashing into the floor. This also allows him to run on walls. He is also shown to use reduced forms of the technique, naming them (X) steps below shukuchi. Shukuchi is revealed to be the reason why he could keep up with Okubo and kill him in his carriage.
- Shuntensatsu (瞬天殺, Blinking Heaven Killer) This is the only attack Sōjirō named himself. It involves using Batoujutsu combined with the speed of Shukuchi. It is named for its ability to instantly kill its target victim, a process that wouldn't even be painful due to the speed with which the target is dispatched. Sōjirō was supremely confident in its ability to demolish Kenshin, but by combining his god-speed with the ōgi used to defeat Aoshi earlier on (Amakakeru Ryu no Hirameki), Kenshin won the battle.
- Shishio Makoto - Mentor and Father figure: Sōjirō first met him as a child when Shishio suffered burns and Sōjirō merely smiled when asked if he was not afraid of death. Once Shisho realized that Sōjirō was abused by his family he gave him a wakizashi as payment for the food and bandages he had received, which Sōjirō used to kill his family once they tried to kill him. Sōjirō learned swordsmanship from Shishio, and regarded him as a combination of both father and god.
- Komagata Yumi - Sister/Mother figure: Sōjirō became close to Yumi who, like his mother, was a prostitute. She showed much concern for his wellbeing, even after he resolved to leave their faction. He had a high regard for her personally.
- Himura Kenshin - An enemy to Sōjirō, since the latter was Shishio's top henchman and wanted to end Kenshin's life with his own hands. Eventually, after a hard-fought battle, Sōjirō loses to Kenshin in their second battle (after a draw in the first) and his adversary tells him that he must find for himself, the correct path in life, the way he himself did.
In Live Action filmEdit
A 5-minute preview of the film shows that Sōjirō was responsible for the kidnap of Kaoru, under the orders of Shishio.
Like the anime and manga, The Legend Ends trailer shows Sōjirō's breakdown to Kenshin.