Friends
Other than the Juppongatana, Soujiro rarely seen in other peoples' company aside from Shishio and Yumi.
I call Shishio Makoto and Komagata Yumi, Soujiro's friends for lack of a better term and should only be taken lightly.
Shishio Makoto
Shishio was the main villain and mastermind behind the entire Kyoto arc. He was also one of the most powerful enemies Kenshin has ever faced.
Before Shishio was obssessed with taking over Japan, he was actually the succcessor of Kenshin as the shadow hitokiri. He was the one called to assassinate Izuka who betrayed the Ishin Shishi and caused the death of Kenshin's first lover, Yukishiro Tomoe. When the Meiji government came to power, they decided to get rid of Shishio to prevent him from leaking anything that could destroy their position.
Shishio was knocked unconcious, covered with oil and burned alive in the forest. Although he was left for dead, Shishio amazingly recovered. The burns however, destroyed his ability to sweat - thus preventing him from fighting for longer than 15 minutes. After recovering, he assembled the Juppongatana, to overthrow the Meiji government.
He was also the closest thing Soujiro could have had as a father. When young Soujiro was forced to kill to save his own life, it was thanks to Shishio's wakizashi which was given as a present, and of course the motto that led his life for the next 10 years.
Shishio may have been a harsh mentor who brought Soujiro into the life of a killer and never really cared for his feelings - even trying to oppress them. Yet he was the boy's savior.
In the end, he met his defeat while battling his predecessor, Himura Kenshin. He had exceeded his time limit of 15 minutes and the heat within his body had begun to build and ended up burning his oils and fats. He died being burnt alive once again.
He is later seen in hell in the company of Yumi and Hoji (who committed suicide to join them) and planning to take over hell.
His last appearance in the manga is when Kenshin sees him in a "vision" during his state of extreme depression at Kaoru's death in the Jinchuu arc. There he is meant to guide Kenshin to hell, but only agrees if Kenshin admits that all he proclaimed to believe in the Kyoto arc was false.
Komagata Yumi
Yumi was a courtesan (highest rank of a prostitute). She is always seen in the company of Shishio, except for when she was made to guide Kenshin, Sanosuke and Saito through the lair of Shishio. She loved Shishio, who returned her feelings. She is always worried about him, especially since he can't sweat.
Yumi perhaps was the closest female companion of Soujiro. She would always joke around with him, even calling him "boy" in a playful way.
Yumi dies during Shishio's battle against Kenshin. Shishio is on the ground as his body overheats. Yumi in worry comes between him and Kenshin, begging Kenshin to spare the man. Shishio takes the chance and thrusts his sword through Yumi and greatly wounds Kenshin.
She is last seen with Shishio and Hoji in a quest to take over hell.