Amnesia/theories

Toritsuki and Haiiroko
Haiiroko/Gray Child, the NPC found sitting on the stairs of the school balcony (along with an appearance in Jar World), can be interpreted as an older sister of Toritsuki due to their visual similarities (especially in haircuts). Haiiroko's location in the school can imply either a desire for or a successful attempt at suicide, which very well could have contributed to Toritsuki's own lost will to live.