Teshima has a close bond with Aoyagi to the point that he can assume what the other is thinking. The two met in their first year of high school (on the very first day, in fact) and have trained together as a team so that they could enter the Inter High together. Since they were both "average cyclists" on their own, they decided to work together in order to surpass their limits. They put in a lot of practice and spent time coming up with strategies, and their hard work paid off when Teshima managed to help Aoyagi be victorious in five out of six races. Though Aoyagi felt guilty about being the only one to get the prize, for Teshima it was satisfying enough to defeat "elite" cyclists. Aoyagi usually calls Teshima by his first name, Junta. Teshima calls Aoyagi by his first name, Hajime, for the first time in Ride 301. After the first Inter High, their bond is even stronger, since now they are captain and vice-captain. During the training camp before the Inter High, Teshima is seen talking with Aoyagi about his condition and chances of winning against Koga. Teshima asks Aoyagi to cheer for him, a request which Aoyagi of course accepts. At first it seems that Teshima doesn't have much confidence, but while racing Koga he sees Aoyagi cheering for him and thinks that he can overcome things he couldn't because there's someone there believing in him. Aoyagi, on the other hand, believes that Teshima's strength comes from within, and that he plays no part in Teshima's ability. He is happy to the point of tears when Teshima wins against Koga. At the start of the 42nd Summer Inter High, Aoyagi helps Teshima calm down and the two prepare themselves for their first and last Inter High competition.
He greatly admires Tadokoro, whom he looks up to as a mentor. Tadokoro encouraged both Teshima and Aoyagi to overcome their limitations as "ordinary cyclists." It was also shown that Tadokoro puts lots of faith into Teshima, especially when Tadokoro said that the second-years would win the race during the training camp for sure.
It's the possibility of riding alongside Tadokoro in the Inter High that drives Aoyagi and Teshima to compete for spots on the team. Teshima has the deep desire to make Tadokoro proud, and is devastated and ashamed when he and Aoyagi lose to the first years, tearfully apologizing to Tadokoro for their failure while he comforts them.
While Teshima typically has a bit of a wry attitude, he is notably always loudly supportive and earnest towards Tadokoro, unabashedly cheering him on during the Inter High.
Being one year apart, Teshima often was in the same races as Imaizumi when they were younger, although Imaizumi was always on the winner's podium and Teshima was left in the crowd. Teshima regarded Imaizumi as an "elite", and insulted him constantly during the Training Camp. However, after losing to Imaizumi, Teshima had to drop the possibility of racing in the Inter High and entrusted his will and the team to Imaizumi and the other first-years. He became supportive of the fellow cyclist along with the rest of the team.
Second year events|Teshima and Ashikiba were close friends in middle school, even promising to reach the top of the cycling world together. However, they are now on different teams and therefore cannot work together any longer. While Teshima seems to still want a friendly relationship with Ashikiba, Ashikiba has decided to treat Teshima as an opponent.
Teshima was friends with Koga, another member of the bicycle club. However, the two had a falling-out over events that took place during the Inter High when they were both first-years, when Koga insisted on racing despite his bad knees, which ended with him falling off a cliff while missing on taking a sharp corner. After their race in the evening of Day 2 of Training Camps as third-years, the two seem to have reconciled after Teshima won, and wore Koga out.
Teshima has gained a lot of respect from Manami after their battles to the Mountain Checkpoint at the 42nd Inter High. From the fact that he kept up with Manami for much longer than anyone anticipated, to showing great sportsmanship by waiting for Manami to catch up, and giving up the eventual victory.