Cut from there to school. Yukiteru Amano, our MC calls himself an observer. He doesn't participate in things, but obsessively keeps track of everything that happens on a diary in his cell phone.
He has no real friends, but rather, imaginary ones.
Hinata is Tenth's daughter and has his diary. She takes Amano's diary at knifepoint.
Aru Akise, another classmate, fakes having a diary and engages Hinata in a high stakes bet. Can he beat future information? Yes. Lying to Amano puts false information in the diary.
Fourth comes for them, but Ninth arranges things so the other cops see what he's up to and arrest him. His diary shows what happens on his cases, so it's useless now.
He asks Ninth for one final favor, and pays with his life, destroying his own diary.
The people who have actually come are not Child Diary users, but Seventh: Ai and Mar are lovers whose paired Love Diaries tell them how to protect each other. (They are both Seventh.)
They see this as a lover's duel: which couple's love is strongest?
Ninth has been captured by a cult. The leader is Sixth, Kasugano Tsubaki.
She has a Dead End showing in her diary. She wants to trade Ninth for Amano's help avoiding it. He's dodged two already.
The problem is that Sixth's diary shows what her cult does...