RinHaru

RinHaru is the slash ship between Rin Matsuoka and Haruka Nanase from the Free! fandom.

Canon
Rin and Haru met when Rin transferred to Haru's elementary school. Rin, who originally went to Sano elementary, wanted to switch to Iwatobi because he saw Haru swimming and instantly thought "I want to swim with him." When Rin first arrived at Iwatobi Haru was at first very put off by him. He didn't understand why Rin wanted him (Haru) to swim in a relay so badly. Rin is shown being very playful with Haru as a child and Haru usually gets flustered by his actions. Rin was finally able to get Haru to agree to swimming a relay, but it was also around the time Rin announced he was going to move to Australia to pursue swimming. Haru was upset by this, but didn't know how to express his feelings about Rin leaving. However, Rin wasn't going to leave until the relay was over, and promised Haru he'd show him a "sight he's never seen before." Sure enough Haru was entranced by the relay and Rin's swimming, but their relationship was cut short by Rin's move.

Rin moved to Australia to pursue his dads dream of being an Olympic swimmer. He hit a wall in his swimming career and became frustrated and upset after realizing that while he might have been decent in Japan, abroad there were many people faster than him. Rin came back during middle school for new years where he and Haru met at the train tracks. Haru was surprised to see Rin and rushed over to him, but the latter looked disheartened and unlike his usual self. Rin then challenged Haru to a race with his intentions being to see if any of his hard work in Australia paid off (since Rin always admired Haru's swimming and wanted to be fast just like him). Haru won causing Rin to become upset. He declared he was quitting swimming, and Haru, thinking he hurt Rin because of the race, quit competitive swimming as well. Haru thought he was what made Rin quit swimming, when in reality it was Rin's own insecurities and thinking he was a failure (with the race against  Haru cementing this idea since he still couldn't beat him) which causes him to declare he was going to quit.

Rin returned to Australia but came back to Japan in High School to attend Samezuka Academy. He meets Haru and the gang again at their old swimming club, right before it gets torn down. He at first acts as an "antagonist" in the first season due to unresolved tension between the two and heavy miscommunication. However, by the end of the season the two have made up and are back to being friends and rivals again.

Fanon
It is considered one of the most popular ships currently in the Free! fandom. The two were shipped from the very first episode. Most were drawn to the childhood friends/rivals aspect of the relationship, and the mystery surrounding Rin and his relationship with Haruka at the time of Rin's first appearance.

The major appeal of their relationship is the fact that they are the only ones able to inspire and ignite each other to do better in swimming. Both of them rely on the other to keep them motivated to swim better and faster so that they can swim on the international stage together. The two are also considered "opposites," with Haru often being reserved and aloof and Rin being emotional and outgoing, so their personalities play off one another's and make for a unique combination.

On AO3 it is the most written ship for Rin, and the second most written for Haru. It is the second most written ship in the Free! tag.

Fandom
AO3

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Trivia

 * Rin's animal is a great white shark, while Haru's is a dolphin
 * Haru only swims free
 * Rin swims free and butterfly
 * Haruka & Rin were the last to get a mook of the two. The mook features darker colors compared to the two's first mook (dark red for Rin and dark blue for Haruka), and is the only mook where the two are touching each other in the image (they are holding each others wrists).
 * Rin considers Haru his "other shining," his first being Iwatobi Elementary School
 * In the Haruka & Rin mook, Mamoru Miyano, Rin's voice actor, calls Rin and Haruka the "destined pair."