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.
They lure Third to the roof of the building, and when he draws his phone to check where they are, Amano punctures it with a thrown dart, killing him.
Deus brings the eleven remaining competitors into a vision and explains the rules: They must discover and kill each other.