as far as we know so far just band mates
he was also a member of the group that jaeyoung was in, the original main vocalist. not sure if they were beyond that
I dont know, but I don't think they where lovers. Maybe a platonic love(?)
based on ch 25, it looks like they've known each other the longest since they joined sunshine ent as trainees at the same time. nothing went on between them if we take jaeyoung's words into account where he claims he's never kissed anyone (ch 9) and that he'd never do romantic things with his bandmates 'cause he considers them family (ch 15). as for what jun thinks... we don't know! but it's assumed he as aspd (ch 25) and seems to have a compulsive tendency to thoroughly control others like jy etc
What is the relationship between happy ( the nonchalant one) and the guy in the last panel? I know he drugged him that causes him to be like that but what is exactly is their relationship? Were they lovers?