Gasai

Yukiteru Amano kept a diary he wrote in each day. He was always the bystander.

Yukiteru Amano (or Yuki) is a loner who never really interact with people and prefers writing a diary on his cell phone with his only companion being an imaginary friend named Deus Ex Machina, the God of Time and Space. However, Yuki soon learns that Deus is not a figment of his imagination but real when Deus makes him a participant in a battle royale with eleven others.