Tadokoro is friends with Makishima and has been since their first year, even though he thinks Makishima can sometimes be pessimistic. Both have a laid-back personality that becomes heated when it comes to races and competition. Tadokoro has a tendency to invite himself to Makishima's food when he visits, despite the rules that no visitors should touch any of Makishima's things, once even eating a whole ham. Makishima tends to think of Tadokoro as a bear due to his large size and equally large appetite, but is bothered by the thought of him losing any weight; he's fond of the thought of a bear riding a bicycle, considering it "cute".
In the manga, Tadokoro awkwardly introduced himself to Kinjou during their first year and expressed a desire to train with him (this was glossed over in the anime where Tadokoro as a first year is less socially awkward). Tadokoro initially sees Kinjou as a rival and declares that he won't lose to him. In Makishima's Spare Bike he is shown looking on as Kinjou is praised for placing high in a race that he also competed in, stewing in frustrated silence.
Despite a somewhat shaky start, Kinjou and Tadokoro are shown to have a deep bond in present day. As vice-captain of the team, Tadokoro has a fierce loyalty to Kinjou and often is seen at his side or helping him with club activities. Tadokoro is notably angry when Kinjou is injured during the 40th Inter-High, as he believed that they would've won that year, but quickly gets over it.
Kinjou had helped Tadokoro, Makishima, and himself develop their bond and balanced their strengths to compliment each other's techniques when riding together.
When they were second-year students, he was shocked that Kinjou crashed during the 40th Inter High and when Fukutomi admitted he was at fault, Tadokoro punched him so hard he fell. Tadokoro believed Hakone Academy was a dishonest team and was brimming with rage long after the race was over. When Fukutomi became captain, he immediately went to Sohoku alone to apologize formally. Toudou mistakenly told Makishima (and Tadokoro) he was starting another fight and to hide him from Kinjou, but Fukutomi actually brought a box of Hakone's Spring-Water-Steamed Buns as a peace offering to Sohoku, which Tadokoro accepted. The next year, Tadokoro recounted the incident to the team's new first-years and how after the visit, he considered Fukutomi to not be a bad guy after all.
During their last Inter High, Fukutomi sees Tadokoro's Human Bullet Train and is impressed, declaring him to be strong.
Tadokoro and Naruko's relationship is notable in that, while they do hold respect for each other, their dynamic is much more akin to a bickering pair of siblings than a typical senpai and kouhai.
Tadokoro acts as a mentor to Naruko, and Naruko often challenges Tadokoro and tries to prove that he can be faster than Tadokoro. Naruko often calls Tadokoro "old man" (ossan), a nickname which Tadokoro dislikes and responds to by calling Naruko "redhead" or "red bean." Naruko strives to become faster than Tadokoro due to his competitive nature and Tadokoro responds in kind, doing his best not to be shown up by a first-year.
Naruko and Tadokoro shared stories of their past losses during the sprinter battle on the first day of the Inter High. Those losses helped them grow stronger since they had to work hard to win. Although Tadokoro won, they contiued to work hard for the team. On Day 3, Onoda told Imaizumi and Naruko to just hang back and admire the power of the seniors before considering retiring from the race, and Naruko was distraught when Tadokoro dropped out from the race from exhaustion after pulling Sohoku level with Hakogaku before the mountainous final stage of the final day.
When Naruko begins pulling the rest of Team Sohoku to meet Hakogaku, it's revealed that, some time before the Inter High, Tadokoro had told him to learn how to climb properly. After arguing back and forward for a time, Tadokoro pulled out his trump card: "casually" commenting on how a sprinter who was also good at climbing would be bound to be flashy and get attention. Despite his claims that Tadokoro would be unable to trick him, sure enough, Naruko practiced climbing and developed his "Armstrong Climb."
Before Onoda joins the club, Makishima notes that Tadokoro seems interested in him. Tadokoro seems to be the first of the third-years to recognize Onoda's potential, cheering for him during the First Year Race and attempting to convince him to get back on his bike after he falls. Just as he does with the other third years, Onoda has respect for Tadokoro.
During the second day of the Inter High, Onoda argues with Makishima to gain his permission so he can wait for Tadokoro to catch up with him. Tadokoro had been losing heart in the absence of his teammates, but after the initial anger he felt at Onoda falling behind with him, he developed respect for Onoda. Despite Onoda's small frame and odd behavior, he was able to bring Tadokoro back to the group. Tadokoro even agreed to sing an embarrassing anime song because it allowed Onoda to ride confidently. He is very grateful to Onoda afterwards, and Onoda is happy that Tadokoro agreed that riding together was fun. Onoda gives him the Love Hime theme song CD after Inter High.
Tadokoro is a mentor figure to both Teshima and Aoyagi. He helped train them and encouraged them as they fought to improve. They both have a great deal of admiration for Tadokoro, and is the possibility of riding with him in the Inter High that pushes them to compete with the first years for spots on the team.
After Teshima and Aoyagi lost to the first years during the training camp, Tadokoro embraces the two of them and gently tells them that they have fought well, but they will likely injure themselves further if they attempt to try and catch up to the others. Teshima and Aoyagi burst into tears and apologize for not being able to make the team, and Tadokoro cries with them.
Teshima and Aoyagi cheer passionately for Tadokoro during the Inter High, crying happily when he takes the sprinter checkpoint and applauding with big smiles when he accepts the green tags. When he runs out of energy on the second day Aoyagi and Teshima attempt to help him catch up to the others, planning to push him until he can reach a decline. While their plan ultimately fails, Tadokoro is encouraged by how deeply the two care for him
Aoyagi in particular looks up to Tadokoro as a sprinter. In his third year, Aoyagi actually takes on some of Tadokoro's defining traits; he creates his own version of Tadokoro's cycling style and even eats like him.
Second Year Events
It's revealed on the second day of the 42nd Inter High that, some time before Tadokoro's graduation, he gave Aoyagi the Love Hime CD he bought in Akiba, telling Aoyagi that since the song helped him during the Inter High, he might be able to find some use for it as well.
Kanzaki Tooji is, in turn, a mentor figure to Tadokoro. When Tadokoro first started out as a cyclist, he was not able to keep up with the other club members as a climber. He came to Tooji and Mr. Pierre with the intention of quitting because he was not good enough to continue, but Tooji encouraged Tadokoro to continue cycling, but convert into being a sprinter. With Tooji's help, Tadokoro was able to develop his own cycling style and become a pillar of team Sohoku. When Tadokoro won the sprint checkpoint over Naruko and Izumida, he dedicated the win to Kanzaki.
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.
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.
The two sprinters spend some time riding together after falling behind. 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 after the Inter High they are seen greeting each other amicably whenever they meet. According to Watanabe, the two of them have exchanged phone numbers.