When they first meet they're opposing each other, him being a member of the port mafia and her a member of the armed detective agency, but they choose to partner together after realizing their current jobs align. It's unspoken, but they know they made a good team, and after completing the job there's a newly grown level of respect between them. She annoys him at first, being too cheerful and friendly for his tastes. But he can't help but hold her in a different regard. Whenever they see each other, there's an unexpected friendship between the two of them. Despite the ADA and the port mafia fighting, they never fight each other. Then when after the current manga arc, for reasons in the current arc I won't spoil for yall people, she is forced to join the port mafia. It hurts her because the ADA is basically her family, ranpo and fukuzawa literally are her family, and she has to turn away and leave for the port mafia. Mori tells chuuya that he's in charge of keeping an eye on her, and she moves into a spare room of his home. She acts alright with it, but the second she's in his house and they're alone, and he knows she's been feeling hurt the entire time, he tells her it's alright to lower the mask and he hugs her as she cries. They become much closer while she's staying with them. He's her companion making her feel better, and they both see new sides to each other. They have long talks, they sit in comfortable silences, they become great friends who trust each other a ton. And chuuya realizes he loves her, and he knows she can't stay. He swallows a lot of his pride and goes to Dazai, saying he knows that Dazai has a plan to get her back and to let him help. He knows mori would be angry if he knew chuuya disobeyed his orders, but he now cares a lot more about helping her. Because he knows this is hurting her. He refuses to let this bright girl be swallowed by the darkness the port mafia dwells in. After she's free from the deal she made and is back home, she knows he helped and thanks him with all the sincerity she can, and they hug. It's warm and tight and full of affection. They see each other more often than before she joined the port mafia, mostly in secret. He recognizes his feelings, but she hasn't quite yet. They keep getting closer quicker now, because they've bonded in a deeper way than before. After their feelings are out in the open to the other, it becomes a question of how to make things progress while being in opposing groups. It's cheesy, but like since B.SD is based around authors, I like to imagine there's a line about Romeo and juliet and rewriting that story. They're enemies, but they're going to do anything to stay together. Their feelings are too deep to turn back at that point, they're incredibly close comfortably.
