CHUUATSU Post-Canon Idea (or I think it could be counted as post-canon):
With Fyodor defeated and the city finally once again settled in peace, the Port Mafia finally calls in their favor: one member of the Agency must join them. Everyone assumes it will be Dazai who will be demanded, after all, he was once their executive, and his mind is a weapon on its own. But to everyone's surprise, Atsushi is chosen.
Atsushi, the last person anyone would have expected to be forced into the darkness of the Mafia. Everyone is devastated, but they can't go back on the deal. They cannot afford to go to war with the Mafia so soon, not when the city is still recovering. And Atsushi, the ever sacrificing one, agrees.
Mori is no fool. He knows Atsushi won't be trusted within the organization, so he assigns him to Chuuya. More specifically, he orders the executive to keep him under constant watch, so much so that Atsushi has to live in Chuuya’s apartment, a decision neither of them is happy about.
At first, they avoid each other completely. They eat at different times, spend their time in separate spaces, and barely speak beyond what's absolutely necessary. Atsushi hates everything the Mafia stands for, and Chuuya doesn't have the patience to babysit someone who refuses to get his hands dirty. The tension is almost suffocating for both of them.
But then something changes. Avoidance slowly turns into small conversations, nothing deep, just a few comments here and there, maybe a little complaint. And then, small chats turn into sitting in the same room without any tension. Into sharing meals together. Into the comfort of simply asking what they should make for dinner. A comfort of having someone to come home to.
It takes a while for either of them to realize it, that the home life both of them dreaded has turned into something domestic. They start waiting for each other before eating. Sometimes, Chuuya brings home extra food because he knows Atsushi will be hungry. Atsushi starts cleaning up because Chuuya rarely does. The apartment, once a place of cold isolation, becomes... warm.
And that's exactly what makes them fall in love. Not the chaos and disagreements of their job, not the danger of the underworld, but the quiet moments at home, where they feel safe. Chuuya realizes he doesn't want Atsushi to lose the kindness that makes him who he is. He doesn’t want his blood Mafia black.
Their home life may be peaceful, but the Mafia isn’t. Atsushi’s refusal to bend to their ways is a problem. He’s still seen as an outsider, a liability, a weak link. There are whispers that he should be eliminated before he betrays them.
Chuuya is stuck in the middle. He still has to drag Atsushi along on missions, still has to cover for him when he refuses to kill, still has to argue. He tries his best to keep Atsushi afloat.
Despite their growing domestic peace at home, Atsushi's place in the Mafia remains uncertain. He refuses to kill, yet his strength makes him an undeniable asset. While most still don’t trust him, Mori keeps him around, watching, waiting, testing.
Everything shifts when a mission goes south. Civilians get caught in the crossfire, and while Chuuya moves to handle it the Mafia’s way, Atsushi steps in and stops him. He saves them without killing anyone, proving that he doesn’t need to play by the Mafia’s rules to win.
Mori takes notice. So does Chuuya.
It becomes clear: Atsushi won’t break. He won’t bend to the Mafia’s ways, no matter how much pressure is put on him. And instead of being frustrated, Chuuya finds himself relieved. He doesn’t want Atsushi to lose his kindness, to become numb to death.
So when the rest of the Mafia starts questioning where Atsushi truly stands, Chuuya makes his choice.
He stands beside Atsushi.
