Spoiler... (only read it if you want, ignore it if you dont want cuz I am pretty sure this part is getting translated within 2 weeks)
His previous manager (Who died), garam and he both are pretty similar. (A lot actually). Well they had a strong relationship (I cant say romantic since they didnt date) but they loved each other a lot. The messy haired( his manager) also played the piano very good, and also couldn't hate heejae like garam. Well, as yerim said, heejae is indeed looking for something (I.e. a sound similar to and as beautiful as his manager) and he found garam. I am not sure if they were romantically involved since I dont remember them kissing. But yeah, garam and the messy haired are very similar but dont worry, he doesn't fall for garam because he is similar to that guy (just partly). He is only freaky in bed with garam. (Prof is like a dog, he bites a lot)
OHHHHHHHH SO ITS HIS MANAGER? NOT ONE OF HIS STUDENT INTERESTING
bro who does the professor keep picturing when garam is right infront of him he should be drooling