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.

The Sacred Eye Temple Incident
The day after Yuki retreats to his house after seeing the corpses in Yuno's house, Kurusu appears at his house with Yuno. He explains Minene was captured the previous night by the Sacred Eye religion, led by the priestess Tsubaki Kasugano, revealed to be the Sixth diary owner.

Upon arrival at the temple, Kurusu is shocked when he learns Tsubaki is a diary owner and considers calling for backup although Tsubaki calms him by explaining she does not wish to become god. When Tsubaki reveals she has a Dead End on her diary, Kurusu supports her request to make Yuki her miracleworker until the time of her Dead End passes. Ai Mikami and Orin Miyashiro try to assasinate Tsubaki with an explosion, Kurusu racing down into the depths of the temple to sort out the faulty sprinkler systems. He succeeds and calls for backup, learning from Yuki of the situation upstairs.

Kurusu eventually finds Minene in the temple's dungeon, learning from her of Twelfth, Yomotsu Hirasaka and his powers of hypnosis. Yuno kills Yomotsu, Tsubaki revealing her true intentions. Kurusu and Minene hide out on a rooftop and form an alliance, Kurusu promising Minene access to police files in return for aiding him in finding other diary owners. Minene is allowed to escape and Kurusu takes control of the temple after Yuki kills Tsubaki. Kurusu and Minene keep in touch over the next few weeks, Kurusu contacting Minene to rescue Yuki and Yuno from the Fifth diary owner Reisuke Houjou.

Turning Traitor
Kurusu eventually learns (via manipulations by Muru Muru) that his son Yoi has a terminal heart disease and will die within three months. Asking Minene to protect Yoi in case anything happens to him, Kurusu investigates the culprit behind the recent animal attacks, identifying him as the Tenth diary owner Karyuudo Tsukishima and shoots him from behind after breaking into his house. Now wishing to become god to save his son, Kurusu ends his alliance with Yuki and Yuno, framing them for killing Tsukishima.

Yuki and Yuno are taken to the police station for interrogations by officer Masumi Nishijima. However, Kurusu plans to shoot the pair and stage it as self defence. Yuki is sent into a room with Kurusu who quickly drops his facade and forces Yuki into a game of Russian Roulette, knowing the last bullet in the gun will kill Yuki. However, Yuno barges in and shoots Kurusu multiple times before fleeing with Yuki. Kurusu survives the attempt on his life, having worn a bulletproof vest following his diary's guidance, but his left ear was blown off. Kurusu issues a manhunt for Yuki and Yuno, putting Minene on alert.

Yuki and Yuno coincidentally spot Kurusu's wife, who they met previously at a bridal fair, on her way to visit Yoi in hospital. The pair are caught by Minene but Yuki accidentally sets off bombs which alerts Kurusu to their location. Kurusu calls Minene and threatens to kill her unless she co-operates, promptiny Minene to abandon Kurusu and form a new alliance with Yuki and Yuno, capturing his wife and son as her trump card.



However, Kurusu refuses to negotiate with her and sends in a swat team to capture Yuki, following afterwards to personally kill him. Yuno charges in, kills the swat team and threatens to kill Kurusu using a grenade given to Yuki by Minene, unaware that it is a flash grenade and will not cause an explosion. Kurusu lets Yuki go but Yuno grabs him to detonate the grenade and take both of them out, although they all survive the flash grenade and the explosion caused afterwards by Minene. Kurusu confronts Yuki, beats him and then holds Yuno at knifepoint. Yuki raises a gun but Kurusu dares him to shoot Yuno. Yuki realises he loves Yuno and manages to shoot Kurusu in the stomach.

Kurusu collapses but rises again to kill Yuno, only for Minene and Nishijima to enter. Nishijima calls Kurusu out for his own crimes, having legitemate evidence to clear Yuki, Yuno and Minene of their crimes. In the anime, Kurusu's conviction is extended when Minene reveals she recorded his original request to guard his son and since he has caused a crime, he is no longer a police officer and thus his diary no longer will work.

Realising the error of his ways, Kurusu asks if his family were saved, Nishijima confirming they were evacuated before Minene's bomb exploded. He asks Minene to continue guarding his son, in exchange for his own life, and he promptly snaps his diary in two. Minene agrees to his deal. Kurusu begins to disappear, but concludes apologising to Yuki for his actions would not be enough to earn forgiveness and he tells Yuki to survive and win the game.

In a final act, Kurusu leaves all of his investigative files to Nishijima, including the recovered diaries of those eliminated from the game.

The Third World
Kurusu is seen alive when Yuno, Muru Muru, Yuki and Minene perform a time leap to the new third world, landing two years into the past. Minene calls Kurusu on a public telephone, anonymously telling him of his son's future disease and tells him to get him treatment immediately before hanging up. Kurusu is left surprised, but heads out to deal with the disappearance of the third Yuno, her father, and the explosions at Sakurami Elementary School.

Kurusu, Nishijima and Mrs. Gasai go to the school, Kurusu chasing Mrs. Gasai inside when she spots her husband. Unsure of the strange goingson around him, Kurusu confronts Yuno as she struggles to come to grips with her feelings for Yuki, eventually firing a bullet at her. Yuki and the second Muru Muru escape Yuno's Illusion World, Muru Muru blocking the bullet. Kurusu watches in silent confusion as Yuki and Yuno reunite, and Yuno stabs herself to ensure Yuki wins the survival game.

Two years later, Kurusu's son is shown to have recovered and the family are seen among the happier characters in Sakurami City.

Trivia

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