Makoto has known Haruka for his entire life, having met when they were just months old, and is his best friend. As a consequence of being together for so long, Makoto is able to read Haruka's mind, understands Haruka's good points, and is often forced to speak as his spokesman. When Haruka doesn’t want to speak anymore, he just "leaves it up" to Makoto to speak for him. It allows Haruka to act aloof and stoic, but still get his thoughts across - though they aren’t stated to be his, necessarily.
Haruka, in particular in his 12-year-old portrayal in High☆Speed!, is rather spoiled by his best friend. He is extremely used to having Makoto always there to help him with making decisions and even speaking up his mind, and when he feels like Makoto oversteps his boundaries, he dismisses him in his mind as a meddlesome guy, but of course still values Makoto.
Both have had near-drowning experiences; In the sixth grade, Haruka fell into a river and was saved by Makoto. During the training camp on the deserted island, Haruka saves Makoto after he fails to save Rei. When Haruka brings Makoto to shore he is terrified when he realizes Makoto is unconscious, and is about to perform CPR, but then Makoto coughs up water and regains conciousness. He tries to sit up and look for Rei, but Haruka tells him that he should be resting and holds him down. Shortly afterwards, Makoto tells Haruka that he is the biggest reason he still swims and gets close to the ocean because Haruka does, and he cares that much for Haruka and what Haruka loves, enough to put his fear aside so he can enjoy it with him, wanting to share the experience, to understand what makes Haruka the happiest, and so he overcomes fear for it. Makoto wants to understand and feel the same emotion as someone else to the point of conquering a deep-rooted fear — something that gave him a panic attack, made him stop moving in the ocean during a storm, while trying to save someone, despite himself.
Haruka greatly dislikes it when people use his girlish first name, but Makoto always somehow manages to lose the argument and calls him 'Haru'  Makoto is also the major person to restrain Haruka from randomly stripping around or diving into fish tanks. When he gets scared, he tends to hide behind Haruka or cling to him in some way.
They are very close and care for each other greatly. In the beginning of the series, Haruka didn't express himself as much, although Makoto always knew what he was thinking. Throughout the show, Haruka begins to speak his mind more often. Haruka cares for Makoto and does not wish to lose his friend In the hotel, he tells Makoto "I appreciate you being here for me, before he leaves for his run.
It is shown that Haruka bought Makoto four aquarium fish. Nagisa told Makoto to name them Nagisa, Rei, Haruka and Makoto, however Haruka told him that it's better to name them Mackerel, Bonito, Tuna and Jackfish.
As Makoto and Haruka become third-years, both of them have to think about their futures. Makoto is very worried about his friend, since the two of them will probably have to part their ways. Makoto is unsure of what to write on his career path questionnaire while Haruka simply writes "Free", showing he doesn't care about getting a job or continuing education. When Makoto asked him about his future plans, he said that "as long as he can swim, he doesn't care", leaving Makoto smiling and commenting how that sounds just like him.
As pressure on what he's going to do in the future keeps on growing upon Haruka, he distances himself from others and doesn't speak his mind as he used to. Makoto sees this but doesn't say anything, not wanting to bother his friend up until they go to the festival together. He then asks Haruka what he's going to do after high school and whether he found a dream for himself. Haruka gets mad at his friend as Makoto was the only person he though would understand him and not ask him the same question everybody else did. They argue and Haruka tells Makoto he has no right to tell him about dreams and future as Makoto himself doesn't have any. Makoto then tells him that he has and that he's going to attend a university in Tokyo. Haruka is stunned and runs away, leaving Makoto at the festival alone.
After their very first fight, Haruka is broken and spends his whole morning wrapped in his bed because all of his recent fights with both Makoto and Rin has brought him down, as well as all the pressure and stress from high expectations put on him. Rin soon comes to pick him up and tells him they're going abroad. In Australia, Haruka keeps quiet and doesn't talk much, making Rin immediately know that he had been feeling overwhelmed and had a falling out with makoto. When Rin and Haruka come back to Japan, Haruka apologizes to Makoto, Nagisa and Rei and tells Makoto that he believes Makoto would make a great swimming coach.
Haruka and Nagisa are childhood friends and were in the swim club together. After the swim club was closed down, Nagisa went to a different middle school from Haruka and Makoto. Nagisa admires Haruka's swimming and enrolls at his school just to swim with him again.
Haruka thinks of Nagisa as a weird guy, whether it is from simplicity or something else.
Haruka and Rei have a good senpai-kohai relationship. When Haruka saw Rei practice pole-vaulting for the very first time, he was amazed by how amazing he looked doing so. When Nagisa told Haruka and Makoto that Rei didn't join the swim club, stating that Rei might hate water, Haruka gained a big dislike for Rei, saying that they don't need someone like that.
When Rei finally met Haruka in person after running to school with Nagisa, he asked Haruka if he was Haru-chan-san Nagisa was talking about. Haruka cut him off telling him not to use both '-chan' and '-san' on his name. After Rei saw Haruka swim at the joint practice with Samezuka Academy, he realized that he wants to have the kind of freedom Haruka does. Haruka told him that it's not freedom that he has, but freestyle. After this event, Rei decides that he wants Haruka to be the one to teach him to swim, although Haru did not want to. After hearing what Rei had to say about freedom, Haru reluctantly decides to help Rei to learn to swim.
In the first episode, Gou visited Haruka's house to ask him for information about her brother, but he didn't answer her, so she decided to head home, running into Makoto and Nagisa. It is shown that she does care a lot about Haruka, as well as other members of the swim club, and is very worried over Haruka and Rin's dramatic relationship. She tries to always be there for Haruka and the others and help them improve their swimming as much as she can.
She is constantly swooning over Haruka's figure.
Miho is Haruka's Classic Literature teacher as well as his homeroom teacher. Haruka's not really impressed by the fact that she became a teacher as her plan B, as well as the fact that she mistook him for a girl, calling him "Miss Nanase Haruka." However, she apologized for it immediately after Makoto had clarified the misunderstanding.
Miho is, alongside Makoto, greatly concerned about Haruka's future after high school. She called him out to ask him what his future plans were after reading his career path questionnaire, only to get "just as it says" as an answer. The teacher tried to convince Haruka to think about the whole thing again, saying that someone with athletic abilities like his could probably enter the sports world easily, but since he kept quiet, she finished the conversation with yet another quote - "Give it a try. You won't know until you do."
- Main article: Rin Matsuoka
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 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 relationship was portrayed to be very tense and passionate (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.
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.
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. 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 then it starts to rain. He later confronts Haruka and Makoto about what they want to do with their futures, with both of them not knowing. Haruka is secretly jealous of Rin, whom is a "dazzling being" in his eyes. He is jealous that he has a dream when he himself has nothing.
In Episode 2 Haruka asks Rin if Samezuka will be in a relay against Iwatobi for the Iwatobi Swimming Club return event. 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. He then later walks away, and the screen shows the light leaving Rin's eyes from when Haruka 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 from pressure to have a dream and future from all his friends and people closest to him. He is curled up in his bed when he hears the door ring incessantly. Annoyed he finally answers it, surprised to see Rin at the door. Rin tells 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'd always admired him. 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 embarrassed and refuses to translate for a curious Haru, instead brushing it off as 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 small quarrel where they argue that its because of each others girly names that they only got one bed. As both boys retort "Your fault" they become closer until Rin realized they have grown too close and turns away, saying he's done fighting and intends to go to the front desk. Rin tells the receptionist that there is some sort of mix up. They however don't get a room change, and must sleep together in one bed. At night Rin can't believe they got stuck in this situation, and Haruka tells him once they're asleep it doesn't matter. Rin agrees and turns the light out. A few seconds later Rin asks Haruka if he's awake, and asking him if he remembered 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". They then get through the rest of 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 states he 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. He asks Haruka what he wants to do, and it shows a picture of Haruka's surprised face. When he steps onto block 6 a member of the National Team steps on text to him. He tells him, "Let's go!" Haruka later 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.
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 swimming together 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: Sosuke Yamazaki
Haruka and Sosuke's relationship in the beginning is very hostile. In episode 2 of Eternal Summer Sosuke confronts Haruka and tells him to "stay out of Rin's way," causing Haruka to have a very negative view of him. However later on in the season Sosuke once again confronts Haruka, but this time tells him to keep swimming his best because as long as he does, Rin will too. In the OVA they are put on the same team. At first they are unsure on how to react to being put on the same team. Later though in the episode Sosuke jumps in front of Haruka, saving him from being hit by the water gun. This moment then leads to a sort of "truce" or peace moment. After this they become friendly with each other.
- Main article: Aki Yazaki
Haru first interacts with Aki after she loses her scarf in the river below the bridge they're both crossing--something that would act as a catalyst to his development. The third character Haru meets as the main POV character of High Speed!, he gives the impression that he doesn't wish to involved in her affairs, stating that retrieving her scarf was "impossible" and leaving. However, it's shown that in spite of Aki seeming her cheerful self again later, worrying over him, it bothers him that he'd coldly brushed her off, and he later gets influenza trying to retrieve it.
Afterwards, although he gradually becomes more open to people, it becomes apparent that he's attentive to her, to the point that Rin believes out of everyone, she and Makoto would be able to convince Haru to join the medley, and thinks "why Haru retrieved the scarf" and "why Aki still wears it despite it having turned brown" is something beyond what he can ask.
- Main article: Goro Sasabe
Goro and Haruka have a good relationship. Goro used to be Haruka's coach while he was still a coach at Iwatobi Swimming Club. When Rin came back from Australia during his Middle School years, Goro was the one to let him and Haruka race back at the Iwatobi Swimming Club pool. He was the only one to know about Rin's and Haruka's race and Rin's breakdown up until he talks with Makoto one night after running into him. He cares about Haruka and others very much and is willing to give his all to help them improve their swimming.