Responses

If they were Japanese, I think it's like when you're so close to someone and you kinda omit -san or senpai or even start calling them with their first name. You wouldn't start calling your close senpai "nii-san" - 'cause that's kinda weird. But yeah, it's literally like that in Korean.
Well, I think Doyun usually called Chan "hyung" behind his back though. (▰˘◡˘▰)
sorry but what's the difference between sunbae and hyung? Is it like 'senpai' and 'kouhai' or.. Σ(っ°Д °;)っ