Takumi and Natsuki were originally friends, presumably having met in the soccer club that Natsuki was head of, however their relationship deteriorated after Takumi punched Miki, her boyfriend at the time, for bragging about his exploits with her. The two of them later reconnect in their last year of high school and become friends again. Both of them are romantically interested in each other and on several occasions they have kissed, despite this, Takumi did not consider the two of them to be dating, and even said to his friends that Natsuki had never expressed an interest in him (although this may have been him lying). Their relationship once again deteriorated when Takumi found out about Natsuki's relationship with a salaryman, an event that caused Takumi to become angry with her and he distances himself from her. In the winter of their last year, Natsuki again reconnects with him, despite his objections. They spend much of the winter together, eventually confessing that they love each other before (presumably) sleeping together. The two separate not long after their graduation, as Natsuki heads to Tokyo for education and Takumi stays in Gunma to pursue a racing career. The two are not shown to ever meet again, though they did exchange letters a few times and Takumi did think about her on several occasions.
Nearing the beginning of Project D's Kanagawa Expedition, a pair of Project D impostors appeared in Saitama, and on one occasion took advantage of a girl named Tomoko. Tomoko's friend, Mika, tracked down the real Takumi and slapped him, not knowing that he wasn't the one who had tricked Tomoko. After confirming with Tomoko that Takumi wasn't the guy she had been on a date with, Mika apologized to Takumi, and the two of them started to meet regularly, starting a relationship. The two bonded over their similar upbringing, with both of their fathers having brought them up to compete in their specific sport. The two become closer, with Mika eventually asking Takumi to call her simply "Mika" instead of "Mika-Chan". The two of them both wish to become professionals in their field, and compete to see who can get there first. Takumi's wife in MF Ghost is heavily implied to be Mika, with her being a professional golfer from Saitama.
Bunta taught Takumi to drive at an early age, and made him do the deliveries for his tofu shop from age 13. Ostensibly this was just so Bunta would not have to do the deliveries himself, although he was secretly coaching Takumi, growing his talent. After Bunta was asked to represent the SpeedStars in a battle with the RedSuns, Bunta tells Takumi to go and race, giving him the first push in his racing career. Bunta secretly modified the car in order to further grow Takumi's skill as he raced against more and more opponents. After the engine blew in the Eight-Six, Bunta replaces it with a racing engine, and gives Takumi half ownership of the car. Bunta later gives Takumi full ownership of the car, buying himself an Impreza, with which he beats Takumi in an impromptu battle on Akina. Bunta then makes Takumi use the Impreza alongside the Eight-Six for deliveries, allowing him to gain a bigger understanding of the Eight-Six's power and shortcomings.
Takumi and Itsuki are childhood friends, having met in middle school, and would later go on to become best friends. The two both got jobs at the Esso Gas Station in Shibukawa during their time at high school. Both of them got their driver's licenses at around the same time, and Itsuki had asked Takumi to go halves on a car with him on several occasions. When it was revealed that Takumi had beaten Keisuke Takahashi, Itsuki originally doubted Takumi's skill, but later went on to become Takumi's biggest fan. Inspired by Takumi, Itsuki buys himself a Eight-Five Levin believing it to be an Eight-Six. Takumi shows Itsuki that even though his car is inferior, it can still perform well if you have the skills. Itsuki sees Takumi as his rival, even though their skills aren't comparable. When Takumi joins Project D, Itsuki and the SpeedStars travel to see him race on a few occasions.
Iketani and Takumi both work together at the Gas Station, and become good friends over the course of his early racing career. Takumi often looks up to Iketani for his car knowledge. Iketani inadvertently kickstarted Takumi's racing career when he asked Bunta to race for the SpeedStars, as this lead to him sending Takumi in his stead. Like most over racers, Iketani is amazed by Takumi's skills, and asks him to take him on a downhill run twice, although he doesn't learn anything for either encounter. Takumi would later race against Mako Sato after Iketani asked him, although Takumi said that he was racing for himself. Iketani and the other SpeedStars travel to spectate several of Project D's races.
A member of the SpeedStars and a close friend of Iketani, Kenji often hangs around the Gas Station. Kenji was originally set to go up against Keisuke Takahashi, however Takumi arrived just in time to replace him. He respects Takumi's skills, and so is present at any race that the SpeedStars are watching.
After being beaten by Takumi in an impromptu battle, Keisuke scours the local area to find the racer that had beaten him, finally being able to meet him when they had their official race. Keisuke, like most racers, is amazed by Takumi's skills and says that his goal is to finally be able to beat him if they ever had a rematch. This rivalry is his driving force behind joining Project D, and encourages Takumi to join so that they can both become the best they can be before they race again. During Project D, this rivalry remains, although the two drivers gain a sense of camaraderie and become friends. Keisuke is shown to still want a rematch at the end of the series, although both Takumi and the Project D support staff are uninterested.
Takumi looks up to Ryosuke because of his reputation as the White Comet of Akagi. Takumi is often modest about himself when talking with Ryosuke, despite his skills and Ryosuke's insistence that he is an exceptional racer. After Takumi beats Keisuke for the second time, Ryosuke believes that he may be the only racer able to take him down, although even he is unable to beat Takumi, causing him to stop racing unless challenged. Ryosuke recruits Takumi to join his new team, Project D, as its downhill ace, coaching both him and Keisuke, and making them into better racers. Ryosuke believes strongly in Takumi's ability to adapt (which leads to the invention of the Blind Attack) and his strong connection with his car (leading to the appearance of the Fujiwara Zone).
Kenta and Takumi have a similar aptitude for racing in the rain, with Takumi having to drive in the rain for his deliveries and Kenta driving in the rain to reduce the wear on his tires. Kenta challenges Takumi to a battle on Myogi, although the battle doesn't last long due to Takumi's sheer skill. Because of his defeat, Kenta keeps training, at one point copying Takumi's Blind Attack (leading to a crash), in hope that he may one day be able to rematch Takumi and win, something that is unlikely. Takumi compares Kenta to Itsuki at one point, likely due to his relationship with Keisuke.
Takumi respects the Project D Staff for the work that they do, and treats them as equals to him. Takumi tries to help out the staff when they are cleaning up the oil and fixing the FD after the crash on Tsuchisaka, being willing to sacrifice his own practice time to do so, however he is told to keep practicing. Similarly, before the races with Purple Shadow, Takumi is willing to give up his place in the inn to one of the team, but is told that he needs the rest more because otherwise he won't perform well.
After Shingo causes Iketani to spin out (and exclusively in First Stage, causes Itsuki to crash), Takumi accepts his request to race in order to put him in his place. Takumi dislikes Shingo for his dirty racing style, and becomes increasingly angry with him over the course of their duct tape deathmatch, as Shingo puts both of them in more danger. Takumi is able to dodge Shingo trying to crash both of them into a guardrail at the end, and he drives off into the night as Shingo collides with the guardrail, breaking his car and his wrist.
Takumi first meets Mako after him, Kenji and Itsuki follow Iketani to his date with her. Takumi is surprised by how attractive she is. Takumi and the SpeedStars later find out that Mako is the fastest racer in Usui., much to their shock. Mako later explains herself to Iketani, and Iketani asks Takumi on Mako's behalf if he will race against her. Takumi accepts, and although Kenji and Iketani dont think Takumi should race her just so that Iketani can get with her, Takumi tells them that he genuinely wants to race her. Mako and her co-pilot Sayuki are both surprised by Takumi's age, and shocked by his skills behind the wheel. After their race, Mako thanks Takumi for the best race she has ever had, and Takumi awkwardly shakes her index finger, clearly flustered by her. Although the two never meet again, Mako is shown to respect Takumi, and even comes to Kanagawa to watch his final battle.
Like Mako, Sayuki is surprised by Takumi's age, and shocked by his skills behind the wheel. After their race, Sayuki displays interest in Takumi, calling him cute. Sayuki invites everyone on a trip to the waterpark, and she spends much of the day with Takumi, at one point going down a waterslide with him, with Takumi having her body pushed up against his back.
Kyoichi explains to Takumi (who he doesn't know has beaten Ryosuke yet) that he doesn't think that the Eight-Six is a viable car for street racing anymore. Despite his beliefs about the Eight-Six, Kyoichi tells Seiji to follow Simulation 3 (follow the opponent until you can figure out their weakness and exploit it), however Seiji, deviates from the plan and subsequently loses. Not wanting this to tarnish Emperor's reputation, Kyoichi invites Takumi to a battle on Akagi. Takumi initially declines but later races with him. Kyoichi reiterates his opinions on the Eight-Six before they race, telling Takumi that he considers the race a lesson and not an actual battle, something he reiterates after Takumi's loss. Takumi later challenges Kyoichi on his home course, and while Kyoichi isn't pleased to race against an Eight-Six again, he is surprised by its newfound power. Takumi is able to beat Kyoichi, earning his respect. At the end of Takumi's race against Kai, he acknowledged that he will one day surpass his once rival Ryosuke Takahashi.
Though he was unaware at the time, Seiji trash-talks Takumi to his face whilst on Akina, leaving him with a poor impression of him. Like many of his opponents, Seiji underestimates Takumi, and deviates from his instructions, ultimately leading to Takumi reclaiming the lead and winning the race. Takumi's victory over Seiji leads to him to finally decide to stop racing on Akina, which eventually leads to his time with Project D.
Wataru first hears of Takumi through Keisuke, and is put into contact with him by Itsuki, who organises a meet up. Wataru and Takumi are kind to each other initially, but they become angry with each other when Wataru accuses Takumi of lying about his engine being weak, as it is a racing engine. Wataru leaves the meet, and Takumi follows him soon after, looking to find out why his car doesn't perform as it should. Wataru is terrified when Takumi takes him on a downhill run, though he is able to figure out the problem with his car, and challenges him to a battle when he gets it fixed. The two battle in a five lap race on Shomaru Pass, where Takumi wins. Wataru later appears as an opponent for Project D in Saitama, although he races against Keisuke. The two hang out at Lake Akina, and Wataru helps Takumi get over his newly developed 4WD complex. Wataru later helps Project D take down a pair of impostors on Sadamine, and watches Project D's final battles against Sidewinder.
Sakamoto is brought in specifically to take down Takumi, and by association Project D. The two racers don't communicate much prior to their race, although Sakamoto approaches Takumi after their battle in order to ask him what his driving force is. Takumi is unable to answer, and Sakamoto says that he is at the professional level, telling him that if he had had his skills at his age, he would be in a much better place. Sakamoto joins the other members of the alliance to watch Project D's battles with Sidewinder.
Takumi's opinion of Ichijo and his teammate Aikawa sours after they cause Keisuke to crash by spilling oil on the course. After Keisuke manages to win his uphill battle, Ichijo threatens Takumi, telling him that they will call in a bosozoku gang if he doesn't lose the race. Though shook by this, Takumi is able to beat Ichijo through his superior braking technique.
Takumi and Joshima both treat each other with respect, clearly impressed by each others skills. Joshima and Takumi participate in a grueling eight lap battle, where both racers are pushed to their limits, with Joshima notably being forced to put both of his hands on the steering wheel, a rarity given his one handed steering method. Joshima reminds Takumi of his father, as they are both skilled veteran racers. Despite technically crossing the finish line first, Takumi considers the battle lost, something that Joshima echoes, as he would have won if he hadn't had to vomit. After their battle, Takumi asks Joshima to take him on a downhill run so that he can see his technique up close. Takumi later practices Joshima's single handed steering, furthering his skill as a racer. Joshima and Hoshino both watch Project D's battles with Sidewinder.
After beating Kyoichi on Irohazaka, Takumi is challenged to a battle by Kai, the son of one of Bunta's old rivals, as Kai had been going after Emperor for a while. The two battle on Irohazaka, and Takumi is shocked by Kai's technique of jumping down from above, taking the inside line of the corner, although he is able to replicate this a few corners later. Takumi is able to come alongside Kai using advice his father had gave him and the two of them become airborne. Takumi is able to win the race when Kai lands on a pile of leaves and spins out. Later, in Kanagawa, Kai and Takumi are able to have a rematch on Nagao. The two are able to stay close to each other for much of the race, however Takumi continually becomes faster and faster, and Kai is lead to spin out when he enters a corner too fast.
Though Takumi is yet to meet him, Shinji spectates his battle with Okuyama, and sees wings sprout from his car. Shinji is initially hesitant to race Takumi, but changes his mind later on. Takumi is amazed by Shinji's skill, especially given his age, noting that he seems to be faster than him despite his car being nearly no different from his. Shinji completely throws Takumi off when he gives him the lead, although he later regains the lead by passing in a corner, reinvigorating Takumi and causing him to racer harder than he had been before, scaring Shinji. Takumi reveals his "wings" when he passes Shinji, which in turn inspires him to not give up. The two battle at their full capacity until the finish, where Takumi's engine blows, causing him to spin out, with Shinji following in tow. Takumi wins the battle.
A close friend of Bunta and Takumi's boss, Yuichi was present for much of Takumi's early career, watching Takumi's races on Akina and reporting back to a disinterested Bunta. Yuichi is kind to Takumi as an employee, and even helps him procure a tachometer after his engine upgrade. Yuichi notes that Takumi and Bunta are rather alike, and is able to get Takumi to race with Takeshi of the NightKids simply by telling him that he wouldn't stand a chance, a tactic that he used on Bunta when they were younger.
Takumi holds a heavy distaste for Miki, as he had often bragged about his sexual exploits with Natsuki. One day, after soccer practice, Takumi had punched Miki in the face for doing this. A few years later, Miki kidnapped Natsuki and intended to assault her, but Takumi comes to her rescue and, although he was unaware of who is opponent was, battled with Miki briefly on Akina and outran him when he crashed the guardrail when Miki's steering wheel jams. He was not seen again after that.
Shiraishi left several anonymous notes for Takumi, telling him that Natsuki had been sleeping with an older man, later revealed to be Shiraishi's father. She calls Takumi's house, and tells him that he can catch Natsuki right now, and he does. Takumi later recognises Shiraishi's voice at their school, and confronts her, with her admitting to it. Shiraishi later gives Takumi some chocolate for valentines, revealing that she had feelings for him.
Tsukamoto was an older student at the high school that Takumi attended. A year after he graduated, he ran into Takumi and Natsuki and offered to take them to Akagi, as he was a street racer there. Tsukamoto tells Takumi about Akina's Eight-Six, unaware of Takumi's racing career. Tsukamoto demonstrates his techniques to Takumi, who is terrified due to Tsukamoto's complete lack of skill. Takumi convinces Tsukamoto to let him drive them back down, as otherwise Natsuki will be sick. On the way down Natsuki asks Takumi to race against a couple in another, much to Tsukamoto's protesting. Takumi obliges and Tsukamoto faints out of fear.
↑Chapter 157 - "We polled all the girls in the third year classes and they chose the coolest guy in school." "You came in second, according to all the third years in class seven."
↑Chapter 502 - "It means he's a master craftsman, and that Eight-Six is his tool." - Kodansha USA version
↑Chapter 506 and 507 are both called "The Craftsman vs. The Athlete"