DazAku

"My hope lies in my credo. A few plain words spoken to me by a certain someone… Those words have let me overcome defeats akin to licking dirt and sipping mud.” - Akutagawa to Hawthorne on his persistent faith"

DazAku is the slash ship between Ryunosuke Akutagawa and Osamu Dazai from the Bungou Stray Dogs fandom.

Canon
Dazai is Akutagawa's former boss and mentor in the Port Mafia. But even today, four years after Dazai left the port mafia, they still behave quite similarly to what they used to. Frequently, Akutagawa is still in Dazai's plans, and Dazai also takes some action to have Akutagawa perform as he desires.

Dark Era
They first met when Dazai was promoted to the rank of executive. Dazai met 14-year-old Akutagawa in a certain forest.(Manga, Chapter 84) He soon invited Akutagawa to join the Port Mafia under his direct commanding unit, and promised to give him anything he wanted in exchange for joining the Mafia. Akutagawa once attacked Dazai with his ability, but his attack immediately disappeared as soon as it touched Dazai, causing Akutagawa to immediately learn how dangerous Dazai is. Dazai, then, conveys to Akutagawa his message that he will not hate him if he declines his invitation, will even give his friends a proper burial, and will provide him and his sister with enough money to live on. But if Akutagawa has vigor for the invitation, Dazai continued, he will gladly grant what Akutagawa seeks. Akutagawa asked if he could give him a reason to live, which Dazai accepted as an answer. Dazai asks for his response. However, instead of replying, Akutagawa just emotionally screamed out. "Good answer," Dazai said with a smile, and then he removed his coat and wrapped it on the shoulders of the crying Akutagawa as a customary symbol of recruitment in the organization, formally welcoming him into the Mafia. (The Heartless Cur (Short Story), BSD Manga, Volume 6),

Under Dazai's direct unit, Akutagawa faced horrific and abusive training to hone his ability, combat skills, and fortitude, usually resorting to harsh and forceful bouts to have Akutagawa refine Rashomon. These include Dazai personally fighting Akutagawa, and despite having the ability to cancel all abilities, Dazai told Akutagawa to fight him back with his ability to train it. Dazai also warned Akutagawa that with only that level of power, he would not be able to survive in the port mafia and also asked if he wanted to go back to being a stray dog in the slumps.(Manga, Chapter 12) In the event of Mimic, Dazai also went so far as to punch Akutagawa in the face and shoot at him three times in order to punish Akutagawa for killing members of Mimic. Dazai then told Odasaku that he saw Akutagawa as a sword without a sheath and that, while he thought Akutagawa had the potential to be the Mafia's strongest ability user, someone needed to teach him how to stow that sword.(BSD LN, Osamu Dazai and the Dark Era)

Present
After Dazai left the port mafia, Akutagawa went to such lengths to find Dazai throughout Yokohama, not knowing the reason for his sudden disappearance. He vows to find Dazai at all costs, (BSD LN, Osamu Dazai's Entrance Exam) and once bombed a public building in broad daylight in the hopes that Dazai would notice, musing on how the latter, who mysteriously vanished, had yet to accept Akutagawa's power. (Manga, Chapter 14) Eventually, Akutagawa crosses paths with Dazai during their bounty hunt for Atsushi Nakajima. (Manga, Chapter 4)

Since then, Akutagawa has shown hostility toward Dazai on the grounds of dereliction of duty, abandonment of his post, and defiance of the Mafia, even threatening to kill him when provoked, while Dazai maintains a distant and cold demeanor toward Akutagawa.During the Agency's and Mafia's conflict, Dazai purposefully had himself captured by Akutagawa in order to obtain information. Throughout their conversation, Dazai maintains his dry humor but communicates with Akutagawa with subtle contempt, even telling his former protégée that his new subordinate is far better than him.(Manga, Chapter 9)

However, Akutagawa continues to seek approval from Dazai whenever the opportunity is given. As a result, Akutagawa is quick to accept almost any personal favor from Dazai, considering it as an opportunity to prove himself to Dazai, while Dazai is also not entirely oblivious to Akutagawa's development. Dazai wants to pair Akutagawa with Atsushi to make them the new Twin Dark, so Akutagawa occasionally worked with his rival Atsushi, even though he was obviously reluctant. This includes the event of defeating Francis Scott Key Fitzgerald in the Guild arc,(Manga, Chapter 36) catching the virus ability user,(Manga, Chapter 51) and also in the event of Decay of the Angel's terrorism outbreak.(Manga, Chapter 83.5)  At one point, Dazai complimented Akutagawa, acknowledged his development, and gave Akutagawa trust.(Manga, Chapter 36)

Akutagawa also remains firmly loyal towards Dazai. Ochi Fukuchi once offered to become his mentor, but Akutagawa promptly turned him down, claiming that no human, not even the war hero Fukuchi, could ever replace Dazai as his mentor. Akutagawa still strives for his approval from Dazai even though he has uncertain time left due to his lung disease.(Manga, Chapter 87)

Fanon
DazAku is a top-ten AO3 ship in its fandom, and also a popular ship is Pixiv, with over 700 works on AO3 and over 4000 works on Pixiv. It seemed to be more popular in the eastern fandom than in the western fandom.

Dazai is, and always has been, the only reason for Akutagawa to live. He was an orphan, used to live in the slums, and also suffered from lung disease. But one day, Dazai completely changed his world. It is obvious that for Akutagawa, Dazai is not only his mentor but his whole world. We cannot define his feeling by any other word than "love." Akutagawa also stated that he is not afraid of death, but what he fears is dying without being acknowledged by Dazai. Even though he knows he has so little time left due to his disease, he is willing to spend the rest of his days gaining recognition from Dazai.

In term of romantic relationship, DazAku is frequently shipped as a one-sided love from Akutagawa to Dazai. Or, in cases where fans want them to be fulfilled in love, it's often the case that Dazai is being kind to Akutagawa, something that hasn't happened in Canon yet. Also, many times this ship was said to be in a toxic relationship because of Akutagawa's overflowing love. There were also cases where fans made them switch roles, for example by having Akutagawa become Dazai's teacher instead so that Dazai could be seen seeking Akutagawa's attention. This is likely caused by a reference to some relationship between the real writers.

Trivia

 * Although they appear to be very different in age from each other because Dazai is in Akutagawa's teacher-like position.They are, however, only 2 years apart in age.
 * In official Bungo Stray Dogs Preschool! from Bungo Stray Dogs Wan!, Akutagawa's profile is written that "love Dazai-sensei the most and wanting to keep him to himself alone."
 * In Anime ED, there are several lines of lyrics that can represent their relationship. Such as "あなたの胸の中で生きている僕がいるのならば(If there's a version of methat exists within your heart)" or "それぞれに今を歩いてる僕らが笑えるように(So that each of us, walking our individual moments, can keep on smiling...)" from Namae wo Yobu yo which can represent Dazai and Akutagawa who walk on their own path while Akutagawa still wanting some place in Dazai's heart. Or in Kaze ga Fuku Machi those lines that "君がくれた言葉は今も僕の中の片隅に置いてある(The words you gave to me are stored in a corner inside me, even now)" are also kind of obvious feeling of Akutagawa to Dazai.