He lives somewhat near the Hikigaya's with his family and sister Saki. He joined the same cram school as Komachi in April of his last year of middle school.
Volume 2 Edit
Taishi is first mentioned as an unnamed boy hanging out with Komachi. Later, Komachi introduces Taishi to Hachiman when he asks for help figuring out what his sister Saki is doing every night. He is worried about his older sister, Saki, who is coming home late every night at 5 a.m., but she gets angry when questioned.
Afraid she might become a delinquent, Taishi requests help from Hachiman and the Service Club through Komachi. After several failed attempts, the Service Club eventually track Saki down and find out she is working late as a bartender under a false name and age. They were unable to convince her to quit. Hachiman asks Saki to meet him the next morning after work.That morning, Saki meets Hachiman in a nearby restaurant with Taishi, Komachi, and the other Service Club members present as well. After Hachiman reveals that Saki's motives for working night-shifts are to relieve financial pressure from her family. Taishi reminds Saki that while she is worried about her family, her family is also worried about her. Hachiman gives Saki information about scholarships so she can go back to studying instead of working and worrying Taishi. Thus, a peaceful relationship is restored between the siblings.
Volume 5 Edit
Hachiman meets Saki by chance at the same cram school during summer break. Saki talks to Hachiman about her brother Taishi. Saki reveals that Taishi really likes Hachiman and wants his advice. Saki wants him to meet Taishi after class, Hachiman only follows Saki after learning that Komachi is with Taishi already.
At a nearby restaurant Hachiman meets Taishi who wants to know more about Sobu, but from a guys perspective. Hachiman tells him all about the requirements to Sobu High school, its benefits, and other high schools as well. When their sisters leave to bring their food orders back to the table, Taishi suddenly asks about the girls at Sobu High in comparison to Yukino who is very beautiful. Understanding the real reason Taishi wanted to talk to him, Hachiman gives a full-fledged explanation of Class 2-F, Yukino's super talented class comprised of 90% girls.
Hachiman felt the necessity of explaining Taishi, that if its about a girl, the girl he likes won't necessarily like him back and advises him to give up (possible reference to Komachi).
When Komachi and Saki return with food, Komachi reminds Taishi that even if he doesn't get into Sobu they can still be friends even if he goes somewhere else, just really good friends, essentially eliminating any romantic hopes Taishi was feeling. When seeing him so dejected, Hachiman decides to cheer him up by saying everyone has their own reason for picking a school (his doesn't necessarily have to be girls). Taishi declares enthusiastically he still has an interest in joining Sobu High school. When Hachiman leaves Taishi calls him onii-san (brother/bro). Hachiman remarks that the chances of that were nonexistent (because of Komachi's own comments). Komachi mentions its still possible if Hachiman marries Saki.
Volume 6.5 Edit
Taishi is only briefly mentioned. During the Athletic Festival Hachiman approaches Saki to make the costumes for the girls event. She had already heard that Hachiman was on the athletic committee from Taishi, who had heard from Komachi, who had pried the information out of Hachiman.
Volume 8 Edit
He participated, along with Saki, in the discussion to help Hachiman come up with a plan to stop Yukino and Yui from running for student council president. Hachiman annoyingly referred to Taishi as "that thing", and does not want his help. Komachi explains that Hachiman may need help or opinions from someone in the same class and decided to ask Saki through Taishi, since Komachi didn't have Saki's contacts, Taishi decided to tag along. Taishi enjoys helping, but is amazed Hachiman's plan to hinder Isshiki's election opponents could be so corrupt. He openly questions Hachiman and pokes holes in the plan.
Volume 11 Edit
Taishi is only mentioned briefly by Hachiman. He is talking to Saki outside in the cold but invites her inside the Service Club so she wont get sick. He thinks since Taishi would be studying with Komachi and taking the same test, it would be best if there were no germs in the Kawasaki household. Hachiman remarks he might have to "kill" Taishi if he gets Komachi sick.