Age: 14 years old
Occupation: Middle School Student
Alternate Name: First
Likes: Having friends, writing in diary
Height: 161 cm
Weight: 53 kg

The main male protagonist of the series and the First Diary user. Also known as Yuki (ユッキー, Yukkī), he is a 14 year old middle schooler who is a rather quiet boy with an anti-social behavior who doesn't talk much. He mostly sees things around him as a bystander and writes it down in his cell phone diary. Yuki becomes involved in the Diary Game when to his shock that Deus, a god-like being whom he thought was an imaginary friend turns out to be real, selects him as one of twelve contestants in a 90 day battle royale where they must kill each other using their special diaries where the last survivor becomes Deus' new heir. Yuki's diary is called the Random Diary, which written from his point of view, gives a detailed explanation of his surroundings very accurately whether it be the present or the future. While versatile in any situation, the main disadvantage of the Random Diary is that it doesn't tells him what is happening to himself which leaves him vulnerable. Furthermore, since the diary is written from his point of view, Yuki can be tricked into thinking something he sees is true without realizing it which is a disadvantage for other people who use the Random Diary.

Unlike most of the contestants, Yuki doesn't want any part of the game as he just wants to live peacefully and doesn't want to kill anyone, but is forced to play to survive and will only kill as a last resort as he is the main target by the other diary users because Deus sees Yuki as his favorite to win. Because he is weak, in order to protect himself, he quickly makes friends and allies to help him including Yuno Gasai, a girl in his class who is madly in love with him and is another diary user. Yuki is kind person yet is naive for quickly trusting people whether they be friend or other diary users as they may have ulterior motives. The most complex relationship he has is Yuno, while Yuno is a formidable ally, her mad obsessive love for him to the point she is willing to kill or use anyone so both of them can survive scares him yet despite the many things she does that shocks him. Despite this, Yuki still relies on Yuno in order to protect himself.

Originally, Yuki's main goal was to find a way to end the game peacefully without killing anyone but that goal quickly changes when during the middle of the game, his mother is accidentally killed by his father, who is also later killed by agents of John Bacchus, the Eleventh Diary user. Grieving over his parents death, Yuno seduces and convinces him that in order to bring his parents back to life, he should win the game and once he becomes the new God of Time and Space, he can use his divine powers to bring them back from the dead. With this new goal, Yuki becomes determined to win no matter what including lying, killing, backstabbing and destruction. His new behavior shocks his friends as his and Yuno actions cause a series of terrorist attacks across the city.
He is named after Jupiter, the Roman god of the sky, weather and lightning.