Keigo Kurusu

Keigo Kurusu is the Fourth diary owner, and initially introduced as an ally to Yukiteru Amano and Yuno Gasai. Chief inspector of Sakurami City's police force, Kurusu uses his "Criminal Investigation Diary" to predict crimes he will investigate, using it to identify other diary owners.

Personality
Keigo Kurusu is a police officer, so it follows with reason that his diary relates to his work. The "Criminal Investigation" diary gives Kurusu information of future crimes and activities committed anywhere within the prefecture he is assigned to. Whether or not this means his diary can record events that will happen simultaneously in different locations is still up in the air. Its major flaw is a location issue; his diary is rendered useless at predicting anywhere outside his patrolling district.

Calm, cool and collected, Kurusu does not show any interest in wanting to win the game. Unlike the other players, his participation is an effort to stop the murders that will occur if it continues. He makes an effort to recruit some of the other diary holders into his cause, but his main goal is to capture the ones too dangerous to be left roaming free. He was chosen by Deus to balance out the criminals and maniacs that are also taking part in the game.

He often acts laidback, calmly challenging Minene Uryu when she bombs Sakurami Elementary School, and gives Yuki the confidence to face Minene. However, he is shown to have his flaws, becoming quite desperate and angry to win the game when he learns his son Yoi will die within three months from heart disease, performing some dirty deeds and breaking the law multiple times in his attempt to win the game, leading to his own elimination.

The Future Diary Alliance
In the first world and version of the survival game, Kurusu formed the Future Diary Alliance with Yuki and Yuno. He survived the other diary owners until the trio were left alive, Kurusu learning of his son's terminal disease and turned against Yuki and Yuno until he was eliminated. Deus Ex Machina had selected Kurusu to balance out the game's players between the sane and the criminals.

In the second world, Kurusu was investigating Takao Hiyama and Minene Uryu's crimes before being given his diary by Muru Muru. After Hiyama's death, Kurusu turned his attention to Yuki, knowing Minene would target him next. In the first meeting of the diary owners, Kurusu promises Yuki that he will protect him. During Minene's rampage at Sakurami Elementary School, Kurusu listened in on her demands and learnt of all of her weapons through his diary. As Yuki was surrendered to her by the students, Kurusu made his move and approached Minene and Yuki, introducing himself to Yuki and explaining the boy was merely bait used by Minene to lure him in. Kurusu aims his gun at Yuki when Minene threatens to kill everyone in the school, Yuki tearfully accepting his fate to die.

However, Kurusu lowers his gun and tells Yuki to pluck up his courage and stop Minene, advising him to cover up his fear and anxiety with a smile. Yuki follows Kurusu's guide and manages to defeat Minene by puncturing her eye. Kurusu tells to Minene to surrender, but she uses smoke bombs and her Escape Diary to flee on a motorbike. Later, Kurusu admits he does not wish to play the game and forms an alliance with Yuki and Yuno to capture the other diary owners who may pose a threat to them.

Kurusu sends Yuki and Yuno on a date to the amusement park so they can act as bait to lure out Minene. However, she does not show and the two go to Yuno's house unharmed. Kurusu has a police guard Yuki's house, unaware of his encounter with the corpses in Yuno's house.

Trivia

 * He is named after Mercury — Roman god of messangers, merchants and travelers.