Osamu Dazai/Relationships

Doppo Kunikida
Kunikida is Dazai's partner in the agency. Though most of the time, Dazai likes to tease and make Kunikida angry to the point of ruining his ideal schedule for a day, they trust each other and acknowledges Kunikida as the future president for the agency.

Chūya Nakahara
Dazai and Chuya were former partners when Dazai was still part of the Port Mafia, forming the team 'Soukoku' or 'Double Black'. They were well known as Yokohama's vicious duo and 'the criminal world's worst enemy' both inside and out of the Mafia, though they both expressed distaste for being forced to work together under Mori's orders.

Their first encounter was on Dazai's search mission for the previous Mafia boss's apparent resurrection, stepping into Sheep's territory. Chuya kicks Dazai to the ground and demands to know the Mafia's intel on Arahabaki, who is rumored to be connected to the return of the previous boss. Dazai refuses, instead of telling Chuya to kill him instead, and recognizes him as the 'king' of the sheep, calling him a 'cocky, over-confident child' while Chuya retorts with calling the 'suicidal brat' Dazai a 'condescending little prick'. In their ensuing fight, the previous boss appears and decimates the area, and Dazai manages to capture Chuya, but at the cost of a broken arm.

Chuya is then taken to the Mafia headquarters, where Mori offers him to affiliate with the Mafia by investigating the previous boss' reappearance, holding the Sheep members as a hostage. Mori then orders Dazai to find the person responsible for the events, making Chuya his partner, much to the pair's displeasure and irritation, beginning their partnership. Throughout this, Dazai is shown to resent Chuya's excessive use of violence and over the top methods of defeating the enemy, while Chuya scorns Dazai's suicidal tendencies and more strategic approaches. Chuya is also slightly irritated by the fact that Dazai took Chuya's title of the Port Mafia's youngest executive away from him.

After the events of 'Dragon's head Rush', the pair were dubbed 'Soukoku' after they had defeated an enemy organization in a short amount of time that night. Their partnership dissolves when Dazai suddenly leaves the Mafia, after Oda's death.

Even though the pair often express their mutual hatred and contempt for each other, they definitely have absolute trust in one another, Chuya using corruption on multiple occasions and placing his life on the line believing in Dazai's abilities. Having known each other for so long, they can even predict the other's thinking, logic, and moves, Dazai even stating that he can predict all of Chuya's attacks, kicks, and punches.

Sakunosuke Oda
While Dazai was part of the Port Mafia Oda was his best friend and as Dazai states, the only one who could come close to understanding him. Along with Ango Sakaguchi, he and Oda were drinking buddies. Other members of the port mafia often remarked that it was odd that the lowest ranking member was friends with an executive.

During The Dark Era, it is revealed that the reason Dazai left the port mafia was because of Oda. As Oda lay dyeing he says "Be on the side that saves people. If both sides are the same, become a good man. Save the weak, and protect the orphans. Neither good nor evil means much to you, I know... but that'd make you at least a little bit better."