Soujirou Seta was the illegitimate son of some rich guy whose family treated him more like a servant than a family member. Actually, they treated him more like something that is very pleasant to kick than anything else. Since the family owned a rice distribution company, he was forced to work for them, moving huge sacks of rice here and there. And whenever he screwed up, they would kick him a whole lot.

Anyhow, at first, he constantly got in trouble for crying, so afterwards, he replaced that action with smiling. I can just imagine his family's reactions. "I guess this kid really likes being kicked." Well, whatever. From that point on, he had only two facial expressions: neutral and deliriously happy.

At this point in time, Shishio escaped the Meiji government and wanted somewhere to sleep. Soujirou found him in his family's basement-like-thing, killing some policemen. Shishio then threatened to kill him, since, obviously, he was witness to several murders. Because he smiled instead of acting all freaked out [because, um, at the time, Shishio has burns all over his body, and is bandaged everywhere] Shishio kinda sorta took a liking to him. Night after night, Soujirou gave him food and new bandages.

Then, after a senseless beating, Soujirou went to Shishio and told him what happened. Shishio told him that only the strong survive and the weak die.Then he lent him his sword.

When the five brothers of the household realized that there was a wanted man covered in bandages in the area, and that all the bandages in the house were gone, they suddenly put two and two together, i.e., "Whoa, okay, so this dangerous convictlike person is in need of the very supplies that are vanishing from our house, so... he must be living in our basement and associating with our father's bastard son! Damn him!" So they confronted him. With the mindset that one has to kill in order to live, Soujirou killed them all. Loudly. The rest of the family came to see what the hell happened, and Soujirou killed them, too. Then he went off with Shishio into the happy sunset.

Later on, Soujirou became a really good swordsman. He learned shukuchi, which is really, really fast. As in, faster than Kenshin's hiten mitsurugi.

Shishio's little troupe, of course, was called the Juppon Gatana. There, Soujirou retained his perpetual smile and fighting skills, earning him both fear and respect. He was given the job of disposing of failed Juppon Gatana members.

He fought Kenshin twice, the first time to allow Shishio time for an escape to Kyoto [he breaks the reverse-blade sword], and then as something of a final test before Kenshin finally fights Shishio. In their second meeting, Kenshin had great difficulty not only because of Soujirou's speed, but because he effectively hid all emotion behind his happy smile.