Sayonara, Hero. - Vol. 1 Ch. 1

[MangaDex]

 

[BakedBanana]

I see a banana peel in the MC shoe, what were these bullies thinking? You are supposed to put that in front of her locker so she slips! (Page 1)

But I'm going to make a guess:
The other characters were saying Yasunori was discharged so I'm assuming he was in the hospital. So I'm betting the secret is he's dying from a terminal illness. (Page 7)

Oh, he says it himself at page 37.