Shin Soukoku

Shin Soukoku is the slash ship between Ryuunosuke Akutagawa and Atsushi Nakajima from the Bungou Stray Dogs fandom.

Canon
Atsushi and Akutagawa's relationship is one of the more important ones in the series, mirroring Chuuya Nakahara and Osamu Dazai's relationship, Soukoku. Although they are enemies and in rivaling companies, they team up several times and use their abilities in sync to form an almost unstoppable team.

Atsushi, being Dazai's current subordinate, is the main source of hatred from Akutagawa towards him as Dazai's ex-subordinate. He is taunted by Dazai that he was never, and will never be better than Atsushi, which fills him with pure rage and malice for him.

Atsushi and Akutagawa's first interaction is in the alleyway after being tricked by Ichiyo Higuchi, Akutagawa's subordinate. Atsushi tries to fight back against Akutagawa, but his Rashoumon rips off his leg, making him scream in agony. Akutagawa begins to walk away, but Atsushi activates his ability and regenerates his leg, and a fight between the two ensues, which is soon stopped by Osamu Dazai.

Later on, as Atsushi is spending the day with Kyoka Izumi, a girl he saved from the Port Mafia, he is impaled in the chest unexpectedly by Akutagawa's Rashoumon and thrown into a truck, where he is brought to a cargo ship going out of the country. He tells Atsushi that even though there was a bounty on his head, his previous action was a genuine intention to kill him, but his regeneration ability is what kept him from being able to succeed. He tries to kill him again, but is stopped by Kyoka, who he turns his attention towards. While he is facing away, Atsushi attacks Akutagawa and they enter another intense fight. In the end, Atsushi is able to escape the ship with Kyoka in tow.

After one of Akutagawa's battles in The Guild Arc, the first thing he thinks of is Atsushi's words of not understanding a person's need to be told it's okay for them to live.

Later in the same arc, Akutagawa boards the Moby Dick with the intent to kill Atsushi, who he soon comes face to face with, and they immediately engage in conflict. Akutagawa tells Atsushi that he doesn't need him to try and understand him. He is tricked by Atsushi that Dazai wishes to talk to him, and, distracted, Atsushi gets away from him once more.

On top of the Moby Dick, Atsushi and Akutagawa, initially fighting each other and the enemy at the same time, work together to defeat Francis Scott Key Fitzgerald, the leader of the guild, before he can crash the Moby Dick into Yokohama. They use their abilities to support each other, working together to eventually defeat Fitzgerald and stop the Moby Dick from crashing.

Fanon
The ship is one of the most popular in the fandom, second only to Soukoku. It is the most written ship for both Akutagawa and Atsushi on Archive of our Own.

Fandom
FAN FICTION

Akutagawa/Atsushi tag on Archive of our Own.

Trivia

 * Their main ship name, Shin Soukoku, comes from the fact that when they team up Dazai labels them Shin Soukoku, a stronger version of his own relationship with Chuuya.
 * They only have a height difference of 2cm (Atsushi is 170, Akutagawa is 172)
 * For their height, they are both some of the lightest characters in the show (both very underweight)
 * They are both orphans
 * Despite saying that Akutagawa will kill Atsushi, he has saved him numerous times from death