The Kōsa Council or Kousa Council ( Kōsa-kai), also called the Kōsa Clan, was a subset of yakuza based in Japan. They were initially friendly with the yakuza known as the Washimine Group, but over several years, relations between them deteriorated after the respective deaths of each crime syndicates' leaders. Because of his hatred of the Washimine family, Masami Kōsa used the Kōsa Council's senior status among local yakuza to slowly weaken the Washimine Group's position. However, Hotel Moscow waged a violent and bloody campaign against the Kōsa Council to eliminate them.

The Kōsa Council mediated with other yakuza groups via the Kanto Peace Council. Within the Kanto Peace Council, the Kōsa Council was at a higher rank than the Washimine Group, allowing the former to make decisions against the Washimine Group concerning territorial disputes. In Tokyo, they were known to possess territory in at least Shinjuku City and near Shiba Park. Under him, Masami Kōsa had underbosses and commanders like Jun Teshigawara and Tadao Ishino. Besides having offices and businesses like nightclubs, in addition to controlling retailers, entertainment venues and industrial waste management, the Kōsa Council also had more firepower than other yakuza since they bought machine guns from gangsters in Hawaiʻi overseas.

For years, Ryūzō Washimine of the Washimine Group and the then head of the Kōsa Council were sworn brothers, having an amicable alliance with one another. Five years before the "Fujiyama Gangsta Paradise" arc, the Kōsa Council's head died, so his younger brother Masami Kōsa became the head. Masami despised his older brother as well as the Council's relationship with the Washimine Group. Two years later after the Kōsa elder's death, Ryūzō himself died. Using the opportunity, Masami had his Council interfere in the Washimines' territory due to the Council's higher rank within the Kanto Peace Council.

Meanwhile, Tsugio Bandō, who was indebted to Ryūzō, volunteered to financially take care of Ryūzō's daughter Yukio. Masami assigned a replacement leader for the Washimine Group since he forced a new rule onto the Group requiring that unless the Group could find a blood-related successor, then he could send in a replacement. Bandō then volunteered to take lead of the Washimine Group as the de facto leader, pushing out the hostile Kōsa elements by engaging in more unsavory fields like prostitution and drug dealing, reasoning that not having an occupation was a disgrace to the Washimine Group.

Furious at the Kōsa Council's actions in a last resort, Bandō requested an alliance with the dwindling Japanese branch of Hotel Moscow to intimidate the Kōsa Council so that the Council would treat the Washimine Group as equals. When she and her men arrived in Japan, Balalaika met with Bandō to discuss helping the Washimine Group while expanding Russian mafia influence in Japan in exchange for weakening the Kōsa Council. Already arranged before the meeting was a bombing of one of the Council's clubs during the meeting, literally shaking the meeting room. Bandō's men were surprised, but he approved of Balalaika's good gesture.

At the same time, Balalaika ordered her Vysotniki to kill more Council members in violent ambushes, including massacres and car bombs. Although the Kōsa Council got greatly crushed, many members of the Washimine Group were scared and concerned that Balalaika's overt killings would grab the attention of Japanese police forces. During Balalaika's second meeting with Bandō, her men surveilled one Kōsa Council affiliate, Kenji Matsuzaki, and confirmed him as a target for elimination alongside other members such as the underboss Jun Teshigawara, commander Tadao Ishino, and other members. Furthermore, the Kōsa Council gangsters were so frightened that one member even tried to turn himself in to the police, but Hotel Moscow gangsters blew up his car in front of the police station. In response, the local Tokyo police began to station officers outside the Kōsa Council headquarters, stymieing Balalaika's desire to abduct Masami Kōsa's family members.

After Bandō's failed attempt to kill Balalaika, she decided to focus on eliminating the Washimine Group instead. Seeing the tides change, Masami managed to arrange a meeting with Balalaika to discuss an alliance against the Washimine Group and to cease Hotel Moscow's attacks on the Kōsa Council. At a meeting between Masami, Morozumi, Yukio Washimine and Ginji Matsuzaki, Masami stated that he still would not allow Yukio to succeed as the head of the Washimine Group despite her qualification, but he nonetheless heard her out on her plan to stop the Russians' interference in yakuza affairs. Yukio responded that her clan was doing everything in their power to rebuff the Russians, but Masami asked what she would do to compensate for the Russians' killings of his men. Yukio agreed that there was a problem but she proposed that she had the power to bring their case to the Kanto Peace Council, angering Morozumi enough to slam his hands onto the table and launching food everywhere, but Ginji shielded Yukio from the food. Once the two Washimine affiliates left, Masami assured Morozumi that they could just wait them out since they had an important meeting the next day that would negatively impact the Washimine Group.

After Yukio managed to frame a robbery on some Vysotniki, Balalaika went to the Kōsa Council the next day to discuss a possible alliance despite her men previously murdering Kōsa members. As Rock interpreted for her, Balalaika apologized for her delay, and Masami smiled that their previous fighting was just water under the bridge. Although he extended out his hand, Balalaika smoked her cigar and did not reciprocate, confusing him. Although Masami had agreed to her terms, Balalaika was still hesitant of his gang's firepower capabilities. The quiet Masami confirmed that his men had recently purchased a machine gun from some guys in Hawaiʻi, but Balalaika asked for Morozumi's gun. Impatient with her, Morozumi slammed the pistol flat onto the table and pushed it toward her.

Once examining the gun and tossing it to Boris, she asked if he had another weapon for back-up, and Morozumi hesitantly gave her his Accu-Tek pistol, which made Balalaika burst out laughing. Examining the pistol, she called the gun weak and stated that she had no use for his group, so the two Russians shot Masami and Morozumi dead in addition to shooting their remaining subordinates in the building. After gunfire was heard from the inside, the Japanese police outside barged into the gates, with Balalaika saying that she was done. Boris asked how they would report Kōsa's death to "Pyotr," with Balalaika responding that Pyotr did in fact order her to kill anyone in Hotel Moscow's way. One Japanese policeman, Yasuzawa, tried to arrest her as she exited the headquarters, but Ishiguro held him back since the car that showed up outside to pick her up had a foreign embassy plate. In the end, the Kōsa Council dissolved due to the conflict.