Shinkai and Fukutomi 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.
Arakita and Fukutomi had a disastrous first meeting, as Arakita insulted Fukutomi and even physically assaulted him. After winning a race on his bike against Arakita on his moped, he is the one who introduced Arakita to the world of cycling, got him interested in road races, and won his respect. (See: Arakita's History.) Arakita goes so far to say that Fukutomi is the only one he "acknowledges." Over the years, the two developed a more friendly relationship, and Arakita often works as an assist to Fukutomi during races. In the past, Arakita called Fukutomi "Iron Mask" (tekkamen), but he currently has a nicer nickname for him: "Fuku-chan". When Arakita dropped out of Inter High, he admitted Arakita is indispensable in forming the cycling team. He also let out a slight smile after Arakita tried to imitate him, but that is after Arakita went to sleep.
Fukutomi was the one who caused Kinjou and himself to crash at the previous Inter-High, due to his own weakness at the thought of losing and not being truly strong. After repenting, he and Kinjou develop a more healthy rivalry that spurs them on in their Inter High as third years to be the fastest and the strongest, something Fukutomi needs in order to properly move forward. Kinjou's philosophy on riding for the team and having faith in the team rubs off strongly on Fukutomi when he sees Kinjou's will to ride even if gravely injured, and Fukutomi acknowledges him as someone who is truly strong, the kind of person he strives to be.