Michie Matsumoto is Taki's colleague and assistant director, and taught at Kitauji while Taki's father, Tohru Taki, taught the band before his son. She is Taki's friend and sometimes mentor. Michie and Taki often spend time together after school in the staff room, and they went to the Agata Festival together. Michie also sometimes gives Taki helpful but subtle advice.
Taki hired Hashimoto as a percussion instructor to help his students on the path to nationals. The two of them were best friends in college and constantly got into mischief together, though Hashimoto later conceded that Taki was always the more attractive among the pair.
Taki and Hashimoto share a playful dynamic. When together, they constantly exchange lighthearted jokes and deadpan banter. Hashimoto's loud, brash humor often complements Taki's subtler deadpan remarks. The fact that Hashimoto's mere presence draws forth a brighter side of Taki's personality does not go unnoticed by the students in the Kitauji Concert Band. Kumiko remarks in the light novel that she had never seen Taki in such high spirits prior to Hashimoto's debut.
Prior to the events of Hibike! Euphonium, Taki distanced himself from all his friends and family, including Hashimoto, after the death of his wife. Hashimoto was hurt and worried, and felt helpless as Taki drowned himself in grief. The two did not regain contact for years until Taki asked Hashimoto to help direct the band at Kitauji. Hashimoto said that he was so relieved and happy that Taki had reconnected himself with music that he almost cried, finally seeing his best friend smile again after years of sadness.
During a Hibike! Euphonium seiyuu event, it was revealed that Taki and Hashimoto had a shouting match over which kind of ramen broth was better. It was described by Oumae Kumiko as "a childish argument."
Taki hired Niiyama as a woodwind instructor to help his students on the path to nationals. The two of them, along with Masahiro Hashimoto, were good friends in college. Niiyama was a grade younger than Taki and Hashimoto, whom both saw her as a little sister of sorts. Though she is now married, she occasionally flirts with Taki just for fun, but without any serious intent. However, this leads most of the band to suspect her for a while of being Taki's girlfriend, until Hashimoto reveals that she is already married.
Taki and his family knew Reina and her father, a famous trumpet player, before Reina's enrollment at Kitauji. In fact, their fathers were co-directors for the Kitauji Concert Band for many years. Taki's sister and Reina also knew each other a very long time, so Taki had known Reina since he was a teenager. It is mentioned in the novels that younger Taki used to cheer up Reina when she was a very young child by playing music with her.
In the manga and light novels, Reina has a early morning ritual of visiting Taki's office, hours before classes start, with the excuse of asking Taki for the keys to the band room to practice. Even though she knows that the band room is usually unlocked because Mizore Yoroizuka tends to arrive even earlier, Reina always drops by anyway just to say good morning.
Later in the novels and anime, Reina confesses to Taki that she loves him after the final concert when all the bands/mentors are being honored, but he misunderstands her intent and takes it as a sign of respect and thankfulness rather than romantic interest.
Taki was seen at first by Kumiko as very strict about practicing and going to the nationals, but Kumiko later realizes that it is all to help the band progress as fast as possible. She respects him and believes he is a good teacher.
When Taki cuts Kumiko from an euphonium part in Crescent Moon Dance, he does it as respectfully as he can, without mentioning her name. Later, he offers a second chance to be given to her for the second round of competition, and reminds her to practice hard.
Taki takes notice when Kumiko begins to practice harder, and, like he does with other students, encourages her. In the second light novel, and only in the novel, Taki buys Kumiko and Mizore Yoroizuka each a bottle of sweet green tea when he finds them practicing in the school building late in the evening.
In the anime, and only in the anime, Taki drives Kumiko home on a stormy night, during which he shares part of his wife's story with her. When her umbrella is destroyed by the typhoon winds, he gives her his own.
After Taki fails to reach his own goals of having the band secure a gold award at nationals, he comes close to a breakdown in his office, shaking all over, and tells Kumiko the full story of his wife and the promise he made to her. Kumiko consoles him with a few words, and he thanks her, seeming to feel slightly better.
Taki had a strained relationship with his father, a famous band director who formerly taught at Kitauji. The austere manner in which Taki's father raised his children may be responsible for his son's penchant of speaking harshly to his students. In fact, the relationship between father and son was so poor that Taki entirely avoided the band at the school he previously taught at, because he associated the act of conducting a band with his father. It was due to this that Taki never taught any bands before Kitauji, even though he had taught general music for many years and was well-equipped to teach a high school band.
However, when Taki quit his job and almost entirely abandoned music due to personal tragedy, it was his father who talked him into returning to teaching music, and into teaching band at Kitauji.