Some people think that Chuuya and Dazai might’ve had a better relationship before Dazai left the Mafia, and while I’d like to believe that’s true, I doubt it. I think Odasaku was the only one who understood Dazai, thus he became his only friend.
As a partner, Chuuya knew everything about him, and Dazai knew everything about Chuuya, but it all remained physical and practical. Dazai knows all of Chuuya’s attacks, but other than that? Maybe there was a time Chuuya tried to understand Dazai despite thinking he was weird and annoying, yet Dazai never allowed him to come closer, to move beyond that surface.
Which is why, with Dazai leaving, Chuuya has never gotten the opportunity to get to know his partner. It’s also why, I think, he doesn’t understand why Dazai left in the first place. They know eachother’s quirks, habits, strengths and weaknesses. It’s like they know eachother like the back of their hands, yet not at all.