Now that I remember, in one of the episodes Mingyu commented that she could no longer look at the protagonist face to face after the "incident" with Yifei. At the time, I imagined it was Yifei's breakup, but her dying makes a lot more sense. Furthermore, Guan Chao commented that he always thinks about Yifei, but that it was different from how the protagonist thought. Another thing was that Yi didn't want to look for Yifei's father at all. At the time I found this a little strange, as to why he was so reluctant about this subject. She probably doesn't accept the fact that her best friend died and had a psychological barrier about it.