Watsuki's Character Notes
The following are taken from the English version of the manga published by Viz Media.
The first part is found on page 86, Volume 9.
The second part is found on page 122, Volume 16.
The Secret Life of Characters (26)
"Like Shishio, this character shouldn't be showing up in here just yet either, but I'll again give you another taste, since so many have asked.
Personality-wise, Sojiro's model is Okita Soshi, captain of the Shinsengumi first unit. The name 'Seta Sojiro' comes from 'Okita Sojiro'-- Soshi's name earlier in life. Since the name is the same, some readers have sought a connection between Okita and Sojiro - but let me just put it out there - there isn't one. Okita is just Seta's model.
Okita Soshi is so famous there's no need to speak of him here - there's always a variety of books about him at the stores, if you're curious. He's so popular that, during the first Ruroken popularity poll, he ranked seventh... even though he'd only appeared in a few panels! Watsuki has used Okita as portrayed by [well-known historical novelist] Shiba Ryotaro, but (this time) it's not the Okita of Shiba's 'Moeyo Ken (Burn, O Sword),' but Okita of 'Shinsengumi Keppuroku (Record of Shinsengumi Bloodshed)' he's using... and that Okita has lost an important part of his human heart, making him emotionless and pretty scary. Some Shinsengumi fans are upset that a character modeled upon Okita is appearing as a villain, but since it's the 'Keppuroku' Okita that's the model, I believe it works well, and helps to make Seta a strong antagonist.
Seta is more difficult to draw than first thought. As of present, I've only done about 1/4th of what I've imagined in my mind. I do want to do him justice, as he is a popular character, but...
Design-Wise, there's no real model. Okita was always envisioned by me with the bangs, and so when he came out looking like this, it was no suprise. I also gave him a slightly feminized air... so that a smile would look good on him.
Amendment
"Sojiro - always popular, always a struggle. As has been previously mentioned, although he is based on the Okita Soshi of Shiba Ryotaro's Shinsengumi Keppyroku ("Blood-Record of the Shinsengumi"), I personally found him a difficult personality to convey. On top of that, because this particular story came at the end of the year, I had little extra time to think it over. My editor was a huge help, actually. (He always understood Sojiro better than me.)
Since the historical Okita Soshi dies young, many fans worried that Sojiro will go young, too, but it's always been my plan to have him stick around. Despite having abandoned all efforts to think for himself and letting Shishio handle everything, Sojiro's not the type to wrap things up by fleeing into death--rather, he'll suffer through life, atoning for all that he's done. It's a harsh way of thinking, but as a human being, it's the only choice he can make. (Think of Shishio as a leader of a radical religious group and Sojiro as a believer filled with crazed devotion, as though he's just handed over his brain. Does that put it in a different perspective?)
Though, for whatever reasons (including family and society), life may be a struggle, so long as one if born into the human race, one must never abandon one's efforts to think for oneself...or so Watsuki believes.
Still, whatever it is I may say about Sojiro's personality, no doubt he'll keep smiling, and living that life of indifference, Sojiro, like Anji, is a character I feel I haven't fully realized; I'd like to bring him back, if possible. Not that any plans are in place at the moment..."