Imaizumi was the first cyclist to stumble upon Onoda easily riding his heavy city bike up the "back-gate hill" at Sohoku High, and he was shocked to see him casually singing while doing so. Because of his pride and desire to beat other cyclists, he challenged Onoda to a race, promising to join the Anime Club if he lost. Imaizumi ended up winning, but Onoda's surprising skill lead Imaizumi to suggest that Onoda should join the bicycle racing club. Onoda admires and respects Imaizumi, and Imaizumi gradually returns the same admiration and respect after witnessing the boy's achievements. He was gracious in defeat when Onoda won the King of the Mountains title during the First-Year Welcome Race and vowed to carry Onoda to the goal during the Inter High when Onoda nearly collapsed and retired. During the earlier part of the series, being able to reach Imaizumi's level of skill was one of the main reasons Onoda trained so hard to become a better cyclist.
According to the some bonus clips at the end of each episode of the anime, after hearing Onoda sing the main theme and gift him merchandise, Imaizumi is shown to gradually develop a liking to Love Hime (that he's quite embarrassed about); he even goes to see the movie with Onoda and to an anime convention with him, and he sings the main theme in the bath. When Onoda is questioned on it, Imaizumi confirms that Onoda has sung the correct lyrics in the sauna. He's backed Onoda up before outside of that - believing that Onoda called his mother to record anime episodes for him after Naruko teased Onoda and thought he was calling his girlfriend.
In the second year, both Imaizumi and Naruko helped Onoda be a better senior to the freshmen.
Naruko and Imaizumi have a relationship as rivals that started from their very first meeting on the day of the First-Year Welcome Race, in which Imaizumi won despite both losing to Onoda after the mountain stage. Imaizumi became the leader of the first years. They are often seen competing in and outside of club activities. The two have also been referred to as a "duo", nicknamed "the redhead and the giant" by other students in the school. Of course, the two do not appreciate being seen as a set, and are vocal in their opposition of this perception. Initially, Imaizumi and Naruko had little regard for each other, as they were too wrapped up in their individual success. As the series progresses, they are seen together more frequently and their behavior becomes more playful than truly hostile. During the Inter High, Naruko expresses the desire to win together with the team, Imaizumi included.
After Inter High, Imaizumi suggested Naruko to become an all-rounder, much to Naruko's chagrin, but Imaizumi genuinely thinks Naruko has glimpses of what it takes to become a better climber.
While Kinjou appreciated his leadership qualities and not be intimidated to speak to upperclassmen, as a fellow all-rounder and captain of the team, Imaizumi found difficult to keep up with Kinjou in the first race before training camp where they paired up to race around the school.
Kinjou, while often serious towards Imaizumi (and occasionally getting under his skin to motivate him to do his best), offers him advice and support in the times he needs it most. Imaizumi greatly admires Kinjou.
Miki is a childhood friend of Imaizumi's. When they first met, she instantly shares with him that she adores bicycles. Though Imaizumi is sometimes frustrated by Miki's actions, such as when she invites spectators to his first race against Onoda, they have maintained a friendship throughout the years. Presently, Miki is pleased about the positive changes she has seen in Imaizumi's personality since he has joined the bicycle club.
Imaizumi is a childhood friend of Tooji and his sister Miki. In the anime, Tooji introduced a young Imaizumi to the world of competitive cycling when he let him borrow a road bike from his shop to try out. Imaizumi ended up riding it for two hours and felt true enjoyment, something he lacked previously as he had no social life in school nor interests outside of schoolwork. Thanks to Tooji, Imaizumi started competing in races.
Imaizumi mentions his desire to defeat Midousuji very early on, and seems obsessed with the idea. He suffered a humiliating loss to Midousuji in the past when Midousuji told him that his mother died, causing him to lose speed and effectively lose the race. During the Inter High opening ceremony, Midousuji makes fun of Imaizumi being gullible enough to believe him and calls him "Yowaizumi" (yowai means weak). Midousuji seems to enjoy taunting Imaizumi quite a bit. When Midousuji tells Imaizumi that he never even considered him a worthy opponent, Imaizumi is mortified and even sheds tears. Up until that point, he saw defeating Midousuji as his only goal in the Inter High.
Second Year Events
During the first year welcome race, Sugimoto imagines his participation in the Inter High as Imaizumi's assist, so it is clear just how much he looks up to Imaizumi. Though Sugimoto does not earn a spot on the Inter High team, he is able to ride alongside Imaizumi before the race begins. He tells Imaizumi that it is an "honor" to be able to ride with him before such an important time. He is even able to pat Imaizumi on the back as encouragement like he dreamed of doing, though not before the goal line as he truly wished. Imaizumi jokes that Sugimoto hurt him, something a bit rare for him, before telling Sugimoto to leave it to him.
Teshima and Imaizumi actually rode against each other in several races before ending up on the same team in high school. Imaizumi was never aware of this because, as Teshima puts it, he always ended up on the winner's stage.
Teshima initially harbored a grudge towards Imaizumi for being an "elite"- that is to say, talented. During their first training camp, Teshima is initially teasing to the point of being condescending towards Imaizumi and the rest of the first years. When he and Aoyagi ultimately fail, he gives the first years the task of bringing Tadokoro and the other thrid years to victory.
Teshima becomes a mentor to Imaizumi after the original third years graduate. Teshima is much more friendly towards Imaizumi in their second, though he still often takes whatever opportunity he has to tease him. Imaizumi is impressed by Teshima's growth as a cyclist as well as his hard working nature overall, defending Teshima when Izumida mocks him. Teshima also fixes Imaizumi's tendency to become impatient and dismiss rookies quickly, especially Sadatoki by making him fall behind and observe him, and also follow him as Sadatoki tries to copy Onoda's form.
Imaizumi is initially harsh and cold towards Sadatoki, putting him in a sink or swim situation in regards to whether he'll be allowed to join the team. After Teshima points out his "talent" Imaziumi gives Sadatoki a chance (and a suggestion to imitate Onoda instead of his brother) and is visibly impressed by Sadatoki's ability to emulate Onoda's cadence. After the race, Imaizumi attempts to act friendly towards Sadatoki with praise and a "warm" smile, to which Terufumi and Onoda react with horror. Despite Imaizumi's awkwardness, Sadatoki is appreciative.