Masumi Nishijima (西島 真澄, Nishijima Masumi) is the chief investigator of the Sakurami Police force and the subordinate of Keigo Kurusu.
Nishijima is an honest and friendly police officer, starting off as a rookie but quickly elevated his way to the position of chief investigator in the Sakurami City Police Department, working under Keigo Kurusu in only two years. In the anime, he is shown to be investigating the murders of Takao Hiyama. During the protection of Cardinal Fred Haiman, Nishijima served as head of security. Nishijima is a romantic officer, believing getting married to be a wonderful thing. He is shown to have a crush on fellow officer Natsuko Ooshima, considering the possibility of dating her, but is heartbroken upon learning she was murdered by Minene Uryu. During the survival game, Nishijima at first remains oblivious to the survival game until learning of the goings on from Minene, and becomes an ally to Yukiteru Amano and Aru Akise.
He develops a unique alliance with Minene and eventually falls in love with her through their seemingly fateful encounters. He attempts to help her find a better life, proposing to her and casually manipulates her into agreeing to marry him and start a family, despite her reluctance to do so. However, he eventually succeeds in altering Minene's personality and outlook on life, which caused her to quit being a terrorist. Nishijima's love for her may have adapted from the feelings he presented to her when she was disguised as Natsuko. He is one of the few characters who does not receive a Future Diary or an Apprentice Diary during the story.
- Nishijima's character theme song, shared with Minene, is "Crazy For You" by NIRGILIS.
- It is unclear if Nishijima is aware that his kids has have godlike powers. Although it is shown in both the anime and the manga he knows that the Minene he married is from an alternate dimension and the same terrorist he is hunting down.