Fukutomi and Shinkai have been friends since middle school where they were in the same cycling club as captain and vice captain. They joined Hakone Academy together. Fukutomi was deeply upset when Shinkai rejected to race in the 40th Summer Inter High because he knew Shinkai was an amazing cyclist and because they had made a promise to win the Inter High together; it's one of the few times Fukutomi was seen showing unrestrained emotion. But he was quick to come to an understanding of Shinkai's difficulties and made him promise to join next year's strongest Inter High team. Fukutomi has a lot of faith and trust in Shinkai to the point of believing he'll fulfill his roles even after suffering defeat; he says that Shinkai is strong because he has experienced and persevered through low points in his life. Shinkai seems to know what Fukutomi is thinking or intends with little to no verbal or visual cues needed, and often elaborates to others what Fukutomi has said so that there's no misunderstanding. He seems to read Fukutomi's usually stoic expressions as if there's plenty of emotion present. The two attend Meisou University later on.
Shinkai was one of the club members initially interested in reaching out to Arakita when the latter first joined, curious about why Arakita rode, and quickly started calling him by his first name. His easygoing personality seems to mesh well with Arakita's aggressive one. Drama CDs and post-credits scenes of the anime frequently depict Shinkai hanging out with him and other members outside of club activities. Shinkai does not react negatively to Arakita's constant complaining and insults, like when Arakita pulls him along after he lost the Sprinters Checkpoint and complains about being tired, and calling Shinkai uncool for losing despite his confidence; rather than being affronted, Shinkai thanks him for allowing him to recover. Shinkai knows that Arakita is truly soft under his abrasive nature, and can easily manipulate this when he wants. On at least one occasion, Shinkai resorted to crying rivers of faux tears when Arakita refused to try the food he was enjoying, which successfully guilted Arakita into eating it. He's also gotten Arakita interested in reading mystery novels instead of just manga, and allows him to copy his homework (even though it appears the answers are usually all wrong).
Izumida looks up to Shinkai as a role model due to them being fellow sprinters. He admires Shinkai very much and is angered when Midousuji insults 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 aspires to fill Shinkai's place as ace sprinter after he leaves, and is determined not to let him down.
On the third day of the Inter High, Shinkai and Tadokoro compete at the flat course at Lake Yamanaka, both pulling their teams as much as they can before succumbing to exhaustion and falling behind. Before their race, Shinkai gently teases Tadokoro, saying that the green tags would "look nice framed."
After falling behind, the two sprinters spend some time riding together. Despite being opponents (and despite their fierce competition only a short time ago), Tadokoro and Shinkai are not only respectful towards each other but casually friendly, commenting on how they think their teams are doing ahead and bemoaning how much they both hate climbs. Shinkai calls Tadokoro "Jin-kun" and shares a power bar with him when he finds he has no more left.
It's unknown if Tadokoro and Shinkai knew each other before the Inter High, but afterwards they are seen exchanging amicable greetings whenever they encounter each other, Shinkai continuing to address Tadokoro by his given name. According to Watanabe, the two of them have exchanged phone numbers.
Yuto is Hayato's younger brother. Not much is known about their relationship yet, but Izumida says that the two appear to have similar attitudes.