Of all the members of the Port Mafia, Chuuya Nakahara might be the most powerful of the bunch, and together with Osamu Dazai, they made for an unstoppable duo. But so much power can present as many problems as it does solutions, as evidenced by the story of Chuuya and the Sheep, his former gang, and just one part of the rich history of Bungo Stray Dogs.
Anime viewers were first introduced to the Sheep at the beginning of Season 3 during a 3-episode adaptation of the seventh light novel, "Dazai, Chuuya, Age Fifteen," where Chuuya was shown to be an enemy of the Port Mafia. This story chronicles the beginning of Dazai and Chuuya's reluctant partnership, resulting in the latter joining the mafia's ranks.
The Delinquents
What sets The Sheep apart from many of the factions of the series is how unremarkable their strength is in spite of their contention against such formidable foes as Ogai Mori's Port Mafia. Their members are all quite young, hapless wannabe gangsters who wish to protect their homes from the fear and violence of other factions.
In this way, their motivations are easily understood, and it's hard not to root for the underdogs when their objective is something so earnest in its simplicity. However, it's no secret that their one and only trump card is Chuuya, whose strength has logically placed him as the leader of the group. With his gravity manipulation and overall combat skill, he's a deterrent in himself.
What his fellow Sheep lack in strength, they make up for in their coordination and dependence on one another. While the anime adaptation doesn't dwell on the inner workings of the group, the novels describe a council comprised of thirteen members that need to be consulted before decisions can be made. Chuuya might be the leader, but he still needs to work closely with the others. This ends up coming back to haunt him...
Chuuya's "Betrayal"
The true story of the Fifteen Arc revolves around Chuuya's quest to solve the mystery of the former mafia boss' "resurrection" and its connection to a legend known as Arahabaki, who is revealed to be Chuuya himself. The unfortunate side effect of his reluctant partnership with Dazai was that his own friends in the Sheep began to suspect that he was betraying them by joining the Port Mafia.
While Chuuya and Dazai complete their investigation, the gang plots together to oust their leader. In a poetic turn, the Sheep end up aligning with GSS, the same organization that Chuuya fought against a mere episode prior. What's fascinating about this turn of events is what the story has to say about power and Chuuya's own responsibilities as a powerful gifted.
Most readers/viewers will no doubt find themselves loathing the Sheep by the end of the arc - and for good reason. It paints them and their supposed respect for Chuuya's leadership as entirely disingenuous. As soon as they thought they couldn't claim his power as their own, they cut him off, fearing him and turning to mercenaries in desperation to compensate for their lack of strength.
Funnily enough, it's Dazai who appears to respect Chuuya's autonomy more than the Sheep. During the scene in the arcade, he is the one who tells Shirase and Yuan that it's his choice what he uses his power for. He might have started by mocking Chuuya, saying "It seems that leading an organization is a lot harder than you'd imagine," but to some extent, Dazai understands him.
Why Chuuya Joined The Mafia
At no point in the Fifteen Arc does Chuuya feel like a true leader and this seems very much intentional. We constantly hear that he is the "King of Sheep" and hear about his reputation, but he's constantly pictured apart from the group. The very name "Sheep" lacks bite and Chuuya's strength in spite of that paints him as a literal wolf among sheep - a comparison Dazai happily draws. He doesn't fit into the role one bit.
To start, he's barely content with his own humanity, being the result of an experiment and having very little idea of his origins, much less what he is at his core. He compromises the trust of the Sheep and continues working with Dazai because he yearns to understand himself. Chuuya's backstory is one of the most storied in the franchise, making up not just the Fifteen Arc, but the eighth novel, STORM BRINGER.
While he's never appeared to fit the leader mold, Chuuya has always been fiercely loyal, and it's one of the main reasons he accepted the role of leader from the Sheep. Even after they betray him, Chuuya takes Dazai's deal to join the Mafia in exchange for their safety. But what exactly made him truly choose the Port Mafia, even after all the time he spent being their enemy?
Obviously, Dazai's proposition didn't give him much choice and Mori's promise of closure about his origins was a fine incentive, but certainly, there was more to it. After all, he kneels in front of Mori and swears his loyalty without a hint of reluctance. There are a lot of layers to why he makes this choice, but the simplest is because of what Mori tells him about leadership.
A leader is both the head of the organization and the organization's slave. For the survival and profit of the organization, they gladly put themselves through any manner of filth. They develop their subordinates and place them where they best fit. And if necessary, they use and dispose of them. For the sake of the organization, they take on any act of barbarism with glee. That is a leader. All for the organization, and for the protection of this beloved city.
At this moment, Chuuya comes to understand what he was lacking as the leader of the Sheep, and more importantly, what he was lacking as a person. While the Port Mafia might not be angels, Chuuya has never exactly been interested in being a saint either. He wants to feel human and like he belongs somewhere, and the Sheep were not where he belonged.
The truth is that the Sheep aren't all that important to the world of Bungo Stray Dogs. However, sometimes in storytelling, there is utility in the inconsequential. Chuuya is characterized by loyalty as much as he is by uncertainty and this tale of betrayal and new beginnings shows that no group was a better fit for him than the Port Mafia.