Their master totally had 3 students: Yuming, Kezhou and an unnamed girl that died at a very young age. We learnt this information thanks to the story part of Kezhou's profile.

But we don't know this girl at all. How old she was, how did she die, was she close with the boys, when did she die, how much younger/older she was than them... We literally know nothing. We just know that she died and high-likely knew kezhou&yuming.

My theory is the reason behind her death, directly or not, is yuming. He couldn't protect her or she sacrificed herself for him. This is why yuming left home and now tries to redeem himself, because he regrets.

But, yeah, this theory can be absolutely wrong lol. This is just purely based on my delusions. Being awake at such a late hour causes me to post this bc I no longer have a sleep routine.