Kurin

Kurin is the friendship ship between Kuro and Rin Okumura from the Blue Exorcist fandom.

Canon
Kuro is the demon familiar of Rin's late guardian, Shiro Fujimoto, who died to protect Rin from Satan, and because Shiro chose to keep Rin from the world of demons and exorcists, Rin was never told of Kuro's existence or the strong connection he too shared with Shiro, until the day Kuro became aware of Shiro's passing.

The moment Kuro over heard the guards talking about his master's death he refused to believe it and began to go on a rampage on the bridge, Rin convinces Yukio to join him when he was called onto the scene to assist the other exorcists. Through his demonic blood Rin is able to hear Kuro's voice, but didn't say anything about it until after Yukio and another exorcist explains Kuro's connection to Shiro and how the demon cat began his familiar. Hearing how Father Fujimoto clamed Kuro down without hurting him, along with hearing Kuro's hurt voice saying that he refuses to believe that Shiro isn't coming back, Rin knew that he couldn't allow Yukio to kill him. Rin explains that to Yukio that Kuro hasn't reverted, like everyone else has assumed, and that Kuro's action's are driven from sadness, which is why Rin wants to talk with Kuro before Yukio gets the chance to use the grenade on him. Without using Kurikara or Satan's power, Rin calls out to Kuro as he tells him that he was raised by Shiro and that he has indeed passed away, as Rin knew that Kuro wouldn't calm down until he excepts the sad truth. Kuro didn't want to believe Rin's clams on the upsetting fact before the demon cat charged at him, Rin stood his ground and literary used his head to stop Kuro in his tracks. Like Shiro on the very day Kuro first met him, Rin tells Kuro that he understands how Kuro much have loved Shiro Fujimoto and the news of his death is making him sad, so he too was hurt by it, before Rin asks Kuro to become his friend. Kuro saw Shiro within Rin and was finally able to except the fact that the person he and Rin cared about is gone, as he shrinks back into his small size form; that surprises Rin. While Yukio spoke with the other exorcists Kuro became very fund of Rin, which helps their case as Yukio is able to pass Kuro off as Rin's new familiar.

Kuro moved into the abandoned boy's dorm with the Okumura brothers, where Rin shares his bed with Kuro. One night after Rin meets Shura Kirigakure and was laying awake in bed thinking about her and other people's words about him, Kuro follows Rin when he left their dorm room to get some air before Rin asks Kuro if he wants to play, so Rin could get some swordsmanship training with the wooden sword that Shura gave him. Kuro doesn't join Rin in his classes, but when Rin was working on his fire control training he has Kuro sitting on his lap as he kept on trying to get it right, while Kuro isn't afraid of Rin's blue colored flames and thinks that they were pretty. When Shura informs Rin of their trip to Kyoto, he brings Kuro allow who serves as his only friend while the rest of Rin's classmates, apart from Izumo Kamiki during the train ride, kept their distance from him. Kuro spend the first part of the day on top of Rin's head and shoulders, before resting himself on Rin's back that he placed next to him on the train. When they got to Kyoto Kuro stayed by Rin's side, before Rin's outburst with Ryuji Suguro causes them to be separated from each other.

When Rin and Bon were making their way to the Impure King through the forest, Kuro they came across Kuro who has been searching for Rin and even though Kuro thought that Rin is crazy for wanting to face the giant demon he agrees to take him and Bon to it. Rin screamed with excitement as he robe on Kuro's back to their target, until they made it into the forming demon made castle and found a patch of untouched ground for Bon to create the barrier while both Rin and Kuro make sure that none of the forming demonic fungus reaches him. After the battle was won and all tresses of the Impure King, wakes up to find Kuro sleeping next to his pillow and has had his wounds from the rot demon bandaged up. Once everyone was patched up and back on their feet, Kuro joins Rin and his friends on their day out in Kyoto before they left to fight a demonic Kraken.

Fanon
Fans love the bond and friendship that Rin shares with Kuro, which might be one of the main reasons why there are some fans who compare them with Natus and Happy from Fairy Tail, along with other anime characters that are close with cat-like creatures and have demon-like abilities or connections.

In a few fanfics and fanart, Rin has known Kuro since he was little, back when Rin wasn't aware of his demon heritage or the demonic world Kuro is from or have Rin already being aware of the existence of Demons, when Shiro brings Kuro to the monastery to have the demon cat serve as Rin's guardian and friend. There have even been a few fanart of the two that has Kuro as his humanoid form that was featured in one of the manga's bonuses. Along with there being fanart that features Rin has a black cat or has him wearing a cat styled hoodie that is based on Kuro.

Fandom
FAN FICTION
 * Kuro/Rin tag on FanFiction.net
 * Kuro/Rin tag on FanFiction.net

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Trivia

 * The bonus in volume 5 is of Kuro pawing at a sleeping Rin's tail.
 * The front cover of volume 8 is of Rin riding Kuro, along with the 8th manga volume having an image of what Kuro would look like has a human and how Kuro lost his large fang.