Shinkai Hayato
Izumida looks up to Shinkai as a role model due to them being fellow sprinters and Shinkai encouraging him to train harder and build the muscles on his body when Izumida wasn't producing the results he wanted after joining the club as a first-year. He admires Shinkai very much and doesn't tolerate insults against him. Shinkai is supportive of Izumida and congratulates him for making the Inter High team, pointing out that, as sprinters, they have consecutive zekken numbers. Izumida is proud to have ridden with Shinkai, and aspires to fill Shinkai's place as ace sprinter after he leaves, determined not to let him down. He tried to defend Shinkai after he lost to Midousuji for the sprint checkpoint, but the rest of the team told him to stop talking because the results can no longer be altered.
Tadokoro Jin
Izumida competes with Tadokoro and Naruko to claim the sprinter checkpoint on the first day of the 41st Inter High. Izumida initially acts haughty towards the Sohoku sprinters, bragging about his "perfect body" and making several disparaging comments towards them (notably, when Tadokoro asks why he has left his jersey unzipped, Izumida replies that he only zips his jersey when he's "serious").
As the three of them compete, Izumida is surprised to see that he has severely underestimated Tadokoro and Naruko. He apologizes for his earlier comments and competes with them seriously, but despite a very close race, Tadokoro claims the checkpoint.
After the race, Tadokoro tells Izumida that though he may have had a "polished" body, he lacked the experience of actually competing in races that he and Naruko had, though not unkindly.
Izumida seems to have some respect for Tadokoro after competing against him. He speaks out against Midousuji for insulting him on the second day, and is impressed upon seeing Tadokoro's "Human Bullet Train" in action.
Naruko Shoukichi
Izumida competes with Tadokoro and Naruko to claim the sprinter checkpoint on the first day of the 41st Inter High. Izumida initially acts haughty towards the Sohoku sprinters, bragging about his "perfect body" and making several disparaging comments towards them.
As the three of them compete, Izumida is surprised to see that he has severely underestimated Tadokoro and Naruko. He apologizes for his earlier comments and competes with them seriously, but despite a very close race, Tadokoro and Naruko emerge ahead of him.
Naruko sometimes refers to Izumida as "Eyelashes."
Kuroda Yukinari
Kuroda is Izumida's childhood friend. Izumida calls Kuroda "Yuki" and Kuroda calls Izumida by his first name, Touichirou. When they were younger, Kuroda was always the athletic one, while Izumida was clumsy. However, Izumida ended up surpassing Kuroda when they both joined Hakone Academy's cycling club.
Ashikiba Takuto
Doubashi Masakiyo
Second year events
In the past, Izumida invited Doubashi to ride together when he noticed Doubashi's troubles. He encouraged Doubashi to release his inner strength. Izumida tells Kuroda that Doubashi has an even greater transformation than he does when he sends Doubashi out to take the sprinter checkpoint.
Doubashi clearly respects and admires Izumida a great deal. He looks somewhat flustered when Izumida praises him for taking the sprinter checkpoint and loudly defends Izumida's somewhat strange-looking "death squat."
Kishigami Komari
Second year events
Izumida and Kishigami race each other on the 2nd day of the 42nd Inter High, in a sort of spiritual rematch of the race between Shinkai and Midousuji the previous year. Seeing Izumida open his shirt and show off his muscles, Kishigami loses control over his urges once again and grabs one of Izumida's pectorals.
Izumida is initially shocked and perturbed by Kishigami's behavior and actions, but as the two of them race, Izumida becomes intrigued in Kishigami's fixation with muscles. Kishigami loudly comments on the 'sublime beauty' of Izumida's muscles, becoming distressed when Izumida pulls away too far for Kishigami to keep him in sight, and even refers to Izumida's pectorals by name.
Despite the odd circumstances and their opposing teams, Izumida and Kishigami seem to have something of a sense of respect for each other after their race, built on their mutual interests. Izumida compliments Kishigami on his riding and, perhaps surprisingly, offers to let him feel his muscles once again, though Kishigami declines.