Age: Unknown, Estimated 35 years old
Occupation: Police Officer
Alternate Name: Fourth
Likes: Justice
Height: 183 cm
Weight: 68 kg

Keigo Kurusu is a police officer, so it follows with reason that his diary relates to his work. The "Investigation" diary gives Keigo 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, Keiga at first does not appear to care about winning the game. His goal is only to stop the murders being caused by it. He makes an effort to recruit some of the other diary holders into his cause, including First and Second. As the series progresses however, MurMur gives him information of his sons imminent death before he should have known to make him more active in the game. He was chosen by Deus to balance out the criminals and maniacs that are also taking part in the game.


He is named after Mercury — roman god of merchants and travelers.