Hidden Girl/megami? She says she can't attend school
Rival-chan? Yandere dev said she is most likley just going to be a normal student
Hanako? She isn't at the school yet
Info-chan? she is HELPING you.
again I seriously have no idea
EDIT:alright i now have 2 ideas!
1.its....Rival-chan! I know i said that" Yandere dev said she will most likley just be a normal student" BUT its confirmed that Osana has a sister! so what if thats rival chan? if she would see you trying to harm osana,you would get in ALOT of trouble. the only downside to this thoery is that rival-chan is most likley shorter than Ayano and Osana,and well...the 3rd person is taller than them....
2.its the butterfly affect,lets say a cooking club member has a crush on osana,he always follows her and stuff like that. but then he sees you killing osana. since he is part of the cooking club,he will not only NOT let you join the cooking club and try to attack you,BUT he will ALSO alert amai. then when amai comes to school she will know what ayano did.then after she gets closer to Senpai,she will probably tell him what you did. aka.Game over.
I mean she's super mysterious and she might have said "make her suffer" in anger or frustration. Maybe Info-chan and Osana dated and they broke up and Info-chan doesn't want her dead, just wants revenge for what Osana did.
I agree with Mystiwaii, it's a good theory. It cannot be senpai, as the uniform suggests female. Unless female senpai... probably not. The heroic theory, extremely genius but unsurprising, so that rivals eliminated. Hananko maybe? I know she isn't attending school yet, but.... yeah, probably not.