Why is Chuuya called Chuuya?
The title basically says it. Almost every other adult character, with a few exceptions, is referred to by their family name, whereas Chuuya is universally referred to by their personal name. Whereas Dazai is always Dazai and never Osamu, even when he was a kid. The other exceptions that I can think of are Ango, Fyodor, and Ranpo.
Also, Chuuya doesn't seem to mind. Like at all. The only time he does seem to mind is that time in Wan when he met Atsushi. He seemed more curious than upset by it.
At first I thought it was Dazai's doing. Like a way to be familiar with friends and slightly insult Chuuya. Then other people took it up like Atsushi mentions in Wan. But that doesn't quite fit. Mostly because of Ranpo. In the Entrance Exam, Kunikida was already calling Ranpo by his personal name rather than Edogawa when Dazai first joined