- Main article: Aiichiro Nitori
- Main article: Sosuke Yamazaki
Sosuke joined Sano SC in the third grade of elementary school when Rin was already a member and could swim butterfly. Rin forced Sosuke to join and learn swimming butterfly too and Sosuke would always be left saying "Ugh, just shut up". Since Rin was faster and wasn't saying anything wrong or untrue, Sosuke let him trying to teach him butterfly, even though it greatly pissed him off. Their relationship kept being like that for over a year up until Sosuke became a good match for Rin. Because Rin was the one who taught Sosuke how to swim, things like the timing of their breaths or the way they moved their arms were exactly alike and, as Sosuke once stated, he couldn't seem to shake it, it was just ingrained then. During the time he was at Sano SC, Sosuke and Rin's relation was having competed for being the representative, being great friends.
When Rin said that he wanted to go Australia and when he said that he wanted to swim at Iwatobi SC, just saying ‘I understand’ while he was half shocked, he didn’t try anything like halfheartedly keeping him back or pressing questions. Sosuke kept being someone who understood Rin well until the end.
In High☆Speed!, Rin stated that it’s better if he's not on the same team as Sosuke because they "understand each other too well". He says that Sosuke’s even more theoretical than he is and that they often quarreled. They settled on it that the faster way is always correct, but their competitive spirit wasn’t half-baked anymore. He said that he doesn't hate him. Rather, he still thinks of him as the person who understands him the most, but sometimes it becomes difficult to be around him. When he ends up understanding Sosuke's feelings, he ends up not being able to say the things that he wants to. He could no longer seriously go up against Sosuke. because he was that one friend who got him way more than his other friends, and that made Rin feel uncomfortable. He said that that was not a friend, but like an alter-ego (you don’t like or hate your alter-ego).
Even though Sosuke and Rin didn't see each other in such a long time, they're still great friends. Sosuke likes teasing Rin, as seen when Rin asked him if he's going to stop swimming and has a special type of a handshake with him. Sosuke is very concerned about Rin and his future and greatly dislikes everybody who stands in Rin's way to success, including Haruka, who, according to Sosuke, did "something Sosuke would never do".
- Main article: Seijuro Mikoshiba
Despite being Rin's senior and captain, Rin isn't all that close to Seijuro, often addressing him simply as "Mikoshiba" when he's not around. However, the moment Seijuro appears, Rin addresses him politely as "Captain". in does not see Seijuro to be very responsible and finds that he is a bit too whimsical.However, he still respects him as his captain and fellow teammate. Rin does not tolerate Seijuro's attempts to go on a date with his sister and often stands in his way. When Seijuro names Rin the next captain, it shows that Seijuro shows respect for Rin. He trusts him to run the team well. They display shows of being good acquaintances
- Main article: Momotaro Mikoshiba
- Main article: Gou Matsuoka
In the drama CD, Rin was curious about Gou going out every Sunday, and thought that she went on dates with Seijuro. He even called Makoto and asked him what are they doing on Sunday. After Rin and Nitori stalked Gou, they were surprised that Gou was actually being helped by Hanamura to buy Rin a Christmas present, and that she wrote on the card "Thank you for everything, onii-chan". Hanamura said that she's jealous of Gou to have such a good-looking brother, but Gou admitted that he can be a pain sometimes and that he's never honest with his feelings.
- Main article: Haruka Nanase
In elementary school, Rin was in the same swimming team as Haruka, Makoto and Nagisa. After they won a tournament, Rin went abroad to Australia to study swimming. Haruka constantly remembered how Rin promised him to show him a sight he has never seen before, and held onto that memory. When Rin made a rare return to Japan, Haruka and Rin raced, and the former easily won. Rin was exceptionally disappointed that his hard work was for nothing; this created a rift between them, as Rin refused to take Haruka's extended hand. Subsequently, Haruka gave up competitive swimming because he thought he hurt Rin and made him quit. Later, when they went against one another again, and Rin won without much difficulty, Rin was not satisfied as Haruka was not swimming at his best potential. He urged Haruka to swim competitively, so that they may have a fair race, to which Haruka complied. Rin's best stroke is the butterfly, but his desperate want to race Haruka, who only swims the crawl stroke, led him to focusing on that stroke and whatever distance Haruka was swimming in the competition. Throughout their various encounters, their friendship was portrayed to be very tense and competetive (as quoted by director Utsumi in the Rin and Haruka mook), as Rin was continually preoccupied with the notion of beating Haruka. Rin sometimes got very frustrated with Haruka, acknowledging that Haruka has the ability to showcase that he is an extraordinary and unusually excellent swimmer, and yet Haruka only cared about immersing himself in water. When Rin officially defeated Haruka at the prefectural tournament, Rin claimed that he will now never swim with Haruka again, resulting in Haruka's utter despair. For Haruka, it was not all about winning. Makoto later told Rin that Haruka had thought something else mattered more, which was why Haruka wanted to race with Rin. Nevertheless, Rin did not understand him. Episode 12 21 Rin cries on Haruka. In the final episode of season one, when Haruka saw that Rin was struggling in his individual race, of which he consequently lost, he immediately rushed from the stadium to go look for Rin, prompting the others to follow. They overheard that Rin was taken out of the relay and wanted to quit swimming for good; this instantly had a destructive toll on Haruka, who faltered, fell to the floor, and buried his face in his arms. Haruka longed desperately to swim with Rin again. When Rei assured Haruka that Rin was not particularly angry with him but was emotionally scarred by his own failures and insecurities in Australia, and that his true desire was to swim with everyone again, Haruka was determined to find Rin. Haruka ended up being the first to locate him, having correctly inferred that Rin would be at the tree that looked similar to the one in the Iwatobi Elementary School yard. When Rin took notice of him, he started to lose his sanity because he was not able to swim with Haruka again. Rin shouted at haru, putting himself down, leading to Haruka to tell him that he began to realize why he swims. Rin becomes frustrated and aimed to punch Haruka, but Haruka stopped him, making them both fall and roll on the ground. When Rin saw what Haruka wrote on the ground before the tournament—he wrote "For the team," a phrase Rin had written in their childhood—he started crying, asking him why could not swim with them again. Haruka reassured him that he still could, extended a hand out and told him that it was his turn to show Rin a sight he has never seen before. Episode 12 38 Rin hugs Haruka. At the relay race at the tournament, Rin, at last, got to participate in the relay with his old team again. Rin swam right before Haruka, and when it was Haruka's turn, Rin was captivated. They managed to win, and Rin ran to Haruka, hugged him and cried that it was, indeed, the best sight he could have ever asked for. In the credits, it was shown that Haruka and Rin became great friends once again, as they began to divulge their respective dreams and goals to one another.
In Season 2 Episode 1 Haruka and the others meet up with Rin at Samezuka in an event to send off the 3rd year members. They partake in a free style race, with Samezuka winning. Rin and Haruka later have a freestyle race, with Rin telling Haruka he'll "light a fire in him." The race is tense and both of them seem to be fired up. They tie in first place, setting a new record, to which Rin says that he made Haruka's cold face grow hot. Haruka looks away, seemingly indifferent to his comment. Later when the event is done Haruka confronts Rin, telling him to come to the school pool where they have set up a surprise for him. When he arrives Haruka and the others capture him and stick a Iwatobi-chan hat over him and surprise him by filling the pool with cherry blossoms. Rin however says it's too cold to be swimming, and the others protest, trying to shove him into the pool. However it starts to rain, causing everyone to run for cover. He later confronts Haruka and Makoto about what they want to do with their futures, with both of them not knowing. Haruka looks at Rin, and secretly is very jealous of him, because in his eyes he's a "dazzling being" who has a dream, when he himself doesn't have one.
In Episode 2 Haruka asks Rin if Samezuka will be in a relay against Iwatobi for the Iwatobi Swimming Club return event. Rin agrees, and Iwatobi narrowly wins. In Episode 3 Haruka and the others spy on Rei, only to see Rin is teaching him all the strokes. In Episode 6 Rin and Haruka face off against a 100m event freestyle with Haruka barely winning against him.
In Episode 9 Haruka feeling pressured by everyone stops in the middle of swimming in his event, feeling no longer that the water is free but a cage. Rin later confronts him, telling him to take swimming more seriously, to have a dream. Haruka retaliates angrily, saying that he doesn't have a dream or future, he's not like Rin. Haruka walks away, and it shows the light leaving Rin's eyes when he says this. In Episode 10 Haruka witnesses Sosuke's confession about his shoulder to Rin, therefore leading Haruka to tell his team to put their heart into it (the relay). Iwatobi wins the relay narrowly.
After the rolling credits of Episode 11 Haruka is very depressed from all the anxiety he is experiencing and pressure to have a dream and future from all of his friends and people closest to him. Rin however shows up at his house, telling him they're going on a trip. Perplexed Haruka is very confused, but Rin tells him he's taking him to Australia. Upon arrival in Australia Rin takes Haruka on a bus where they are later seen by a fountain. Rin leaves Haruka for a brief moment, causing him to panic. Rin is shown to just have been getting drinks, and Haruka tells him not to leave again. Rin apologizes, and they both sit on the fountain. Rin later apologizes for his actions at the tournament, saying he ignored how he felt and forced his own feelings on him. He also tells him that he wants him to understand that he has always admired him. After telling him, Rin feels very relieved at finally getting it off his chest. Rin then takes Haruka to the ocean, hoping he'd be more relaxed by the water. He invites him to come in but Haruka refuses, saying he doesn't feel like it. Rin asks him if something happened to him, since he usually would jump at the first sight of water. Seeing Haruka is upset with having his fight he does his best to console him. He then talks about how the ocean made him feel better upon arriving in Australia. When he looked over it he would think that Haruka and the others were just on the other side, as if it some how connected them. He then goes on to tell Haruka about his experience in Australia, how he was very frustrated and sad since even if he was good at swimming in Japan it meant nothing here. He quickly fell behind the other students, hitting a wall with his swimming. Haruka tells him to stop, because he understands that while he was in Iwatobi Rin had seen and done many great things.
Smiling Rin then takes Haruka to meet his home stay parents, Russell and Lori. They have dinner with them, and they comment on how Rin always looked up to Haruka and talked about him all the time. Rin becomes refuses to translate for a questioning Haruka, instead saying it's unimportant and avoiding his questions. At night when arriving at their hotel Rin is surprised to find the room only has one bed. Haruka wonders if its normal for Australia, and Rin tells him its not. They then get into a petty quarrel, witch each saying its because each others girly names that they only got one bed. Rin go to the front desk to clear up the mistake. Rin tells her there is some sort of mix up, but she tells him they're booked full and can not change rooms. They don't get the room change, and the scene switches to Rin and Haruka in their single bed. Rin can not believe the situation they're in, but Haruka tells him it won't matter when they're asleep. Rin agrees, and turns out the light.
A few seconds later Rin asks Haruka if he's awake, asking him if he remembers when he said he admired him. He then proceeds to ask him if he remembered the first time they met. He didn't think anyone could beat him, but when he saw Haruka, all frustration vanished when he thought, "there's someone more amazing than me, I want to be able to swim like him." He explains that's why it's hard for him when Haruka isn't always ahead of him. Without him he has nothing to aim for. Haruka tells him he too remembers. They later reminiscence about the time they set a new tournament record together (earlier in the season). He says that when he felt Haruka coming up next to him, he knew for sure he was going to "enter the same world as him". The boys then get through the night.
The next day when waiting for the train Rin asks Haruka what he would do if he did find a dream. The scene then cuts to Rin and Haruka on the train, their reflections shown in the glass as Rin points out sights to him. Rin takes him to the Aquatic Centre, where the best swimmers in the world race against each other. This is the place he really wanted to show him. He then says they should go swim. They spot the National Team, and Rin says this is where they practice. Haruka is uncertain if they can swim here, Rin reassures him by saying no one with out the guts would swim next to the National Team. They then go down to the pool. He asks Haruka what he wants to do, and it shows a picture of Haruka's surprised face. When Haruka steps onto block 6 a member of the National Team steps on the block next to him. He tells him, "Let's go!" Haruka then imagines what it would be like to stand on this very block with thousands of people cheering in the stadium. He feels the rush once more of swimming and competing, and he reaches his emotional climax when he connects swimming with flying, for they both bring freedom to the wielder. He thinks in this place forever I want to keep swimming. Back in the present Rin is next to him, saying they should go too. Haruka agrees, smiling.
A picture is then seen of two doves (seagulls in the blueray version, but director Utsumi cleared up that they are suppose to be doves) flying in the air together. Towards the end of the day Rin tells Haruka his teacher from Australia was wondering if he'd consider swimming with them. He intends to see him and ask him to formally invite him on the team. He tells Haruka this time he won't lose (referring to his previous failures when he was younger), it's time for revenge. Once done speaking, Haruka says, "Rin, I've found it too," implying he had also found a dream. The scene ends with their backs to the viewer and the two birds seen earlier flying off in the distance, implying the two boys are heading into the same direction (future).
In the beginning of Episode 13 Haruka thanks Rin for taking him to Australia, to which Rin answers with a squinted, whole-hearted smile. After nationals they later join him in a petal-filled pool at Samezuka, where they all swim happily. They are later seen going in their respected directions, with Haruka training at a school in Tokyo (for swimming) and Rin also training in Australia. The season ends with a picture of Haruka and Rin facing off in a professional circuit. Rin smiles at him, Haruka in turn smiles back too. It ends with the starting buzzer going off and him leaping into the air, and "For the future" written across the beautiful blue sky.
- Main article: Makoto Tachibana
In the High☆Speed! novel, their relationship is detailed a little further, impacted by an incident by the riverbank, where they save Haruka as their friend Aki rushes to call the ambulance and several contacts. After Haruka is hospitalized, Rin and Makoto walk to the bus stop together, with Rin expressing anxiety. Makoto tries to reassure Rin that Haruka will be fine, smiling--but Rin reveals the person he was scared for was Makoto, who, when they saved Haruka, couldn't stop shaking.
Afterwards, Makoto feels a responsibility placed on him by Rin to ask if Haruka would join the relay with them, and once they do, Rin and his relationship continues to build. From the riverbank incident on, Rin keeps a subtle eye on Makoto, at one point jumping in to calm him when he seems to have trouble with the water, and Makoto comes to find it easy to joke around with and tease him, and freely laughs at others' joking about him. As seen in the beginning of Free!, late in High☆Speed! Makoto realizes and becomes apprehensive over whether Rin will stay in Iwatobi or go some place else, and determinedly calls Rin by his first name--the second to last to do so--cementing their bond during these years.
When Rin returns to Japan from Australia, he ignores Makoto and Nagisa in favor of challenging Haru, brushing them off with "You're still hanging out with these guys Haru?" Makoto, still remembering their childhood bond, converses with Coach Sasabe worrying over why Rin had become so distant, and calls him afterward asking that he join Samezuka's swim club so that they could see each other some day soon. When Nagisa confirms Rin joined, he's very happy. And sometime later, despite Rin's supposed dismissal of him, when Gou mentions to Rin that Makoto came with them to the ocean, a flash of anxiety for him passes by his face again, showing that he too remembers their bond after all. He wakes up in the middle of a storm worried about Makoto, and in the morning runs to the beach to check if he and the team are safe.
It's only after Rin wins against Haru at regionals that the Iwatobi boys meet him again, bumping into him while looking for Haru. When Rin assumes Haru is upset due to losing against him, Makoto chides him, reminding him that swimming meant something more to Haru, and he's pretty sure that Rin was the one who taught him what it was. In the finale, after Haru and Rin heal their rift, Rin rekindles his friendship with the boys, able to laugh with them again.
- Main article: Nagisa Hazuki
- Main article: Rei Ryugazaki
Rin initially does not think highly of Rei when he first sees him swimming a relay with the boys. He instantly becomes jealous and does not think he swims well. Rei then confronts Rin at Samezuka in episode 11, asking him why he joined the relay, what happened to him in Australia, and what he really wants from Haruka. Rin gets defensive and asks why he has to answer these questions and also questioning Rei why he wants to swim in a relay. Rei tells him the same answer Rin gave when he was younger, saying he desired to swim with his friends and be the best team around. Rei then leaves. Further on in the episode Rin contacts Rei and tells him the answers to the rest of his questions.
- Main article: Aki Yazaki
Unlike Makoto and Nagisa, Rin calls Aki the more formal "Yazaki-san." In High Speed!, Rin thinks Aki is someone who can influence Haruka's way of thinking, and although he later feels bad for asking that Aki prompt Haru to join the medley because of this, he's reassured that it wasn't offensive, because much like him, she wants to see Haru join the medley too. They have a mutual respect for each other, connecting through their conversation about Haru, and are even able to joke around together.
In Aki's POV, she's shown to wonder over how he's always "running towards somewhere," seeking something.