Masumi Nishijima

A police officer and subordinate of Keigo Kurusu. A main character in "Mosaic." During the investigation of Minene, he becomes involved in the Future Diary incident after being paired with Natsuko Oshima, who was actually Minene in disguise. During the confrontation between Yukiteru and Kurusu, he learns the truth about the series of events from Minene and helps to invalidate Kurusu's investigation diary by removing his authority. He also cooperates with Yukiteru and the others in place of Kurusu, concerned about the dangers of the Future Diary.

In "Mosaic," he once confessed to Minene while she was disguised as Natsuko, and he does not regret it. During the battle with the 11th, he confesses to Minene again, this time proposing with a wedding ring. Minene, taken aback by his forceful proposal, agrees to marry him if they destroy HOLON and defeat the 11th, leading them to work together. They head to the Twin Tower Building to destroy "HOLON III" and disrupt the 11th's diary by rewriting the "Escape Diary." However, as he rushes to Minene, who had her right arm blown off by a landmine, he is shot from behind and falls, becoming her shield.

In the third world, he has a child with the second world's Minene, who decided to stay in the third world. He accepts that Minene refers to herself as "I" and calls her Minene, indicating that he understands the situation to some extent.