Soukoku

Soukoku is the slash ship between Nakahara Chuuya and Dazai Osamu from the Bungou Stray Dogs fandom.

Canon
Both Dazai and Chūya were paired up with each other when they were 15 years old. The man who had paired them was Mori Ougai, the new chef of the mafia, who reasoned that "only diamonds can be polished by diamonds."

The duo was named Soukoku after one memorable night during the events of "Dragon's Head Rush", where they had defeated an enemy organization over night. This resulted in them recognizing themselves as Soukoku, or Double Black, among other enemy organizations. They are also known as "Yokohama's vicious duo". The pair also have unconditional trust with each other. As stated in Bungou Stray Dogs: Dead Apple guide book, "The core characterization of Dazai and Chūya's partnership is based on pure trust where both of them are capable of leaving each other's life on the other's hand without a second thought or doubt." (pg 54-55).

Their partnership collapsed for four years when Dazai decided to leave Port Mafia and join the Armed Detective Agency. However, their partnership, resumes during the conflict with the foreign organization The Guild. Also, the duo seem to be know each other to a questionable point when Dazai says he knows Chūya's "Thrust", to which, Chūya responds with "I was just going easy on you".

In Dead Apple, Chūya uses corruption to save Dazai, even if Ango says that he's dead for sure.

When the duo falls on the ground Dazai forces Chūya to rest on his lap, because he doesn't want to save him if the fog separates Chūya from his ability.

Fanon
It is believed that Soukoku is a business ship since it earns and buys a lot, however, many disagrees as well.

Trivia

 * During the character designs, the creators of the series mentioned that Dazai and Chuuya would be in a Partner/relationship, as well as that Chūya was originally designed for Dazai.
 * The ship is the most popular ship among the Japanese and Western fandom.
 * Their stage actors ships them (Dazai played by Hideya Tawada and Chūya played by Ueda Keisuke).
 * Dazai and Chūya are 51 days apart in age.
 * Their height difference is 21 cm (Chūya - 160 cm, Dazai - 181 cm).
 * They have a whole episode dedicated to them for their reunion in season two, called "Double Black".
 * Chūya has put his life on the line just to save Dazai many times.
 * During one of the twitter events, Chūya confesses that he is, indeed, in an inseparable relationship with Dazai.
 * Dazai once stated that he did not enjoy the company of men or hugging men. Yet, in one promo picture, he is seen hugging Chūya from the side, while wearing a very amused smile and winking at the other man.
 * Red camellias (Camellia Japonica) are often used to represent them. The said flower stands for love, passion, trust, and desire. In some cases, it also represents death or sorrow of losing someone, and is related to Chūya's ability (For the Tainted Sorrow), where he has a higher chance of dying.
 * The name was canonically given to the pair of men from the boss of mafia, Mori Ougai.