The Juppongatana & Soujirou
Hoji Sadojima
While Hoji and Soujirou are constantly in the company of one another, they very rarely speak to each other. The first time Hoji talks is to tell Soujirou not to interrupt the gathering of the Juppongatana with his treats.
Soujirou, we’re in the middle of something. Later?
He speaks curtly by the nature of his practicality. It seems that he acts this way with all other characters with the exception of Shishio and so this shouldn’t be taken with any other meaning.
Anji Yukyuzan
When Anji and Soujirou meet in the forest near Kyoto, Soujirou uses the honorific -san while Anji calls him “Soujirou-dono“. Taking from the manga glossary:
Honorific. Even more respectful than -san; the effect in modern-day Japanese conversation would be along the lines of “Milord So-and-So”. As used by Kenshin, it indicates both respect and humility.
Anji is generally a respectful character (as seen by his actions during his battle with Sano) and so the use of this honorific is justifiable.
Cho Sawagejo
Cho is never shown personally talking to Soujirou but when he is interrogated by Saito in the jail, he talks about Soujirou. When asked which of the Juppongatana could have killed 50 well-trained men in 1-2 hours, he names Soujirou as one of the two who could have done it. This could be taken as displaying respect for Soujirou’s skill or merely acknowledging it.
Kamatari Honjo
Kamatari and Soujirou interact once in the manga but seem to be friendlier with one another than most others. While Soujirou refers to Kamatari as he does with everyone else (using the honorific -san), Kamatari calls Soujirou “So-chan”. Taking from the manga glossary:
Honorific. Can be used either as a diminutive (e.g., with a small child – “Little Hanako or Kentaro”), or with those who are grown, to indicate affection (“My dear…”)
Saizuchi
Soujirou treats Saizuchi respectfully and addresses him as “Elder Saizuchi”. In their dialogue in chapter 97, it is implied by Saizuchi that only Soujirou treats him that way.
Usui Uonuma
Usui treats Soujirou slightly better than most other characters he interacts with (that is, without contempt). In one scene, Usui regards Soujirou with the suspicion that he is hiding something.
Even with your twisted emotions, I can see right through you. [Usui, ch97]
Fuji, Iwanbo, Henya Kariwa
Soujirou is not seen talking with these members of the Juppongatana but it can be guessed that they would treat one another with respect and civility with some measure of respect for Soujirou’s higher rank as Shishio’s right-hand man and for his ability as a swordsman.