Yukiteru Amano is a 14-Year old boy from Sakurami City who is granted the power to see into the future for up to 90 Days in advance, aswell as 11 more Diary Owners. Deus, God of Time and Space, grants these twelve people the power in order to compete in a Survival Game.
Yukiteru Amano, a junior high student, initially has trouble making friends. He views himself as a spectator from the sidelines and often writes down what he sees in a cell phone diary, which reflects his introverted attitude. Tormented by solitude, Yukiteru interacts with his imaginary friends: Deus Ex Machina, the God of Time and Space, and Muru Muru, Deus's servant.
After the deaths of his parents, Yukiteru's personality undergoes an extreme change. He becomes very tactical and intelligent, using the effects of his Random Diary to their full extent, easily dispatching the men who assassinated his father. He is even able to trick his own diary into presenting him with a password needed to access the bank vault that John Balks hides, speaking loudly about the possibility of ambushing Balks' secretary to gain the password, which the diary produces as his future, since it is based around his own views and beliefs. He transitions from being passive into resourceful as he was able to use a ball to distract Tsubaki Kasugano, thus leaving her defenseless. He also was able to save Yuno from being electrocuted by turning on every electronic device available at the maximum capacity to make them overload and blackout.
However, with his newfound bravery and intelligence comes underhanded cruelty, willingly leading Kamado Ueshita and her orphans into a trap so she can be eliminated, and he uses Minene Uryu and Masumi Nishijima to clear the path to Balks even though Nishijima is killed and Minene loses an arm. He also appears to lack morals, allowing Yuno Gasai to kill people without scolding her, using her psychotic violence to his own means.
However, it becomes clear as the game reaches its climax that Yukiteru still is afraid at heart, confused by his own actions and asks Minene what he should live for if resurrecting his parents is the wrong thing to do. The events that follow with the deaths of all close to him knocks him out of his crueller self back into his kinder, caring personality; eventually becoming completely selfless to ensure everyone in the Third World gets a happy life, particularly Yuno.