Usamarin

Usamarin is the friendship between Usamaro and Rin Okumura from the Blue Exorcist fandom.

Canon
When Rin and his younger twin brother Yukio were little their guardian, Shiro Fujimoto, read them a story about a little demon that is the main reason for a festival that is only celebrated once every eleven years. After Father Fujimoto finished reading the story and told them how old it is, he asks the twins what they would do if they were ever to encounter the demon from the story, Yukio said that he'll fight the said demon while Rin smile as he told Shiro that he would befriend demon. Since he didn't see the demon as a bad one from how the boy and villagers in the story had believed the demon into their village, before the rabbit masked exorcist came across the little demon.

Eleven years later, when it was time to celebrate the festival again, the Phantom Train Rin, Yukio and Shiemi Moriyama were sent to exorcise managed to escape from, all while causing a large amount of damage that destroys and buries a forgotten shrine that frees the little demon that that sealed in it. When Rin first found the demon he mistook him for a human child who might had gotten hurt when the ruble fell, and brought the "boy" back to the Japan Exorcist branch. After the "boy" came to and started to run around the room until Cheng-Long Liu manages to catch him and shows Rin the demon's rabbit tail before stated that Rin had rescued a demon. With it being festival time and many exorcists had their hands full in exorcising demons and straightening the restored protective barriers, there wasn't any time to identify the unknown demon which is why Mephisto Pheles subjected that they kill the demon without a second thought, but Rin disagreed with that idea. Since Rin's well meaning stunt with the Phantom Train has him being band from exorcising demon for the time been until his punishment his up and doesn't want the demon to be killed, Mephisto leads Rin in charge of the demon until the festival is over to which Rin agrees to in order to save the demon from being killed. Yukio, however, disagreed with Rin and when he asked him why he agreed to the ask Rin replied that he didn't have much of a choice.

By the time Rin went back to his drome room he sees that the cage that the demon was placed in has been relocated to his room and had no idea how they manged to do so, from how big the cage is. A few hours later Rin notices the demon looking at him when he took a sip of his water and offered him some and the demon drank the whole thing which gets him wanting to his business, but Rin manages to unlock the cage and get the demon to the toilets just in time. After Rin notices that the demon fells terrible and says his thoughts outload, the demon spoke to him for the first time as he didn't approve of Rin telling him that he stinks. With help from Ryuji Suguro, Renzo Shima, Konekomaru Miwa, Rin is able to give the little demon a bath to get the stink off him, while Shiemi and Izumo Kamiki got the demon some new clothes to wear. As Rin waves goodbye to his friends when they went back on duty, Shiemi informs Rin that the demon had wondered off to which has Rin going after him until he and Kura found him.

Just as Rin was about to place the demon back into his cage he notices that the little demon is hunger and decides to cook them something first. The demon loves Rin's runny dish and after Rin learns that he doesn't have a name, he decides to name him Usamaro. While Rin does his assigned homework that Yukio gave him, Usamaro looks through a box that if filled with toys and the story book that Shiro gave Rin and Yukio when they were kids, before Usamaro shows Rin the book and climbs onto his clap as Rin begins to read it to Usamaro.

Fanon
Fans love the bond that Rin formed with the little demon who views Rin as a friend and possibly a big brother-like figure. That is why most fanart has the two in brother-like moments, some of those drawings include Yukio and created an AU that has Usamaro being able to stay with Rin and become his little brother, like Rin had said to Yukio before a swarm of demons forces Usamaro to sacrifice his freedom to save Rin and the other innocent people.

AO3 currently has 3 fanfics. Tumblr also has a few drawings and posts of the two from the anime film.

Fandom
FAN FICTION