Keigo Kurusu

Keigo Kurusu (来須 圭悟, Kurusu Keigo) is the Fourth diary owner, and initially introduced as an ally to Yukiteru Amano and Yuno Gasai. As the 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.

Diary & Abilities
Kurusu's diary is known as the Criminal Investigation Diary which tells him of criminal activities withing a 90 days interval. This makes it perfect for him to track into many of the other participants as they are mostly sociopaths who engage in multiple criminal activities within 90 days, the time limit of the Survival Game. The diary however can only tell him of his own investigation so he must work to make the best use of the information he can gather.

Being the Chief of Police of Sakurami City, Kurusu has absolute command of the Police force to do his bidding. The fact they can fight for him without knowing of his Diary and his participation of the game, implies he's a very cautious and manipulative individual if needed. He's so skilled in this and so powerful that he in one opportunity deployed a team of special units (seen as SWATs in the manga & anime) to surround the Sakurami Hospital and almost trap Yuki, Yuno and Minene Uryu.

He also is an expert marksman and can gun targets with lethal precission. During Minene Uryu's attack on Yuki and Yuno's school, he was capable of shooting against Minene with enough precision to keep her on bay (if he killed her, a detonator linked to Minene's heartbeat would trigger bombs that would blow up the entire school). He also is fairly cautious and wears a bulletproof vest at all times.

Kurusu is considered by even Deus and Muru Muru to be one of the favorites to win.

Trivia

 * Kurusu is named after Mercury, the Roman god of messengers, merchants and travelers. His surname contains the Kanji (来), meaning 'come'.
 * Kurusu is the only diary holder besides Yukiteru and Yuno to kill another diary holder.
 * Kurusu's character theme song is "Running For Your Life" by Shoukichi Ishida.