Mirai Nikki

Yukiteru (Yuki) Amano is a loner who never really interacts 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. Within this Diary Game, the contestants are given special diaries that can predict the future, with each diary having unique features that gives it both advantages and disadvantages. (Source: ANN)