Hiiro would enjoy all bonding activity regardless of what music it is, because he's just like that and they were apart for so long. It must make him very happy when he is able to spend time with Rinne these days. That said, Rinne pushes Hiiro to be more independent so he might encourage Hiiro to make the music selection. I think Hiiro would like to listen to Rinne's music, but Rinne would be embarrassed by that lol. You know, Hiiro's probably gotten to know older idol music too from Aira. This is good Nii-san Bonding Material. I do not picture the characters of Enstars as existing in the real world with real world figures, so I can't really imagine them listening to any actual artists. I think it's different from other parts of real pop culture existing in the Enstarsverse. Mainly, I don't want to think they inhabit the same world as me because it feels weird to me. Rinne and Hiiro listening to music from artists who are active in the real world feels uncomfortable to me somehow. I like put a certain distance between myself and the characters.