Chuuya knows one of Dazai’s secrets—he has a sibling, a younger one at that. Dazai actually mentioned something about his sibling when him and Chuuya were talking. You’re not actually in the Port Mafia, despite your brother being part of it. Dazai won’t allow you join, due to Mori and that he doesn’t want you to end up like him. Chuuya has actually been keeping an eye on you for Dazai’s sake.

“You know Dazai hates when you’re in here.”

You’re in one of the Port Mafia towers with him.