Masumi Nishijima is a supporting character in The Future Diary who serves as the chief investigator of the Sakurami City Police Department. He begins his career as a rookie officer but demonstrates enough competence and diligence to be promoted to chief investigator within two years, working closely under his superior, Keigo Kurusu. Nishijima is introduced to the story when he questions Yukiteru Amano about a serial killer case, and his involvement deepens as he becomes gradually aware of the existence of the Future Diaries and the deadly survival game orchestrated by the god of time and space.

Nishijima is characterized by his honesty, friendliness, and unwavering dedication to his work. He is a calm and analytical individual who prefers to rely on facts and evidence rather than emotion, maintaining a professional demeanor even in stressful situations. Despite this composed exterior, he is not without emotional depth; he carries some personal baggage from past experiences and shows a genuine warmth and kindness toward others. He looks up to his boss Kurusu with respect but is not blindly obedient, and when he begins to suspect that something is wrong he is willing to question authority and seek the truth on his own.

His primary motivation throughout the series is to bring justice and order to a chaotic situation. After Kurusu betrays the other diary holders, Nishijima plays a key role in investigating his former superior's crimes, eventually exposing the truth. This marks a turning point in his role, as he moves from a peripheral detective to a more central ally of Yukiteru Amano and the terrorist Minene Uryuu. He joins forces with Yukiteru and the detective prodigy Aru Akise to uncover the deeper mysteries of the diary game, and he becomes a trusted collaborator who uses his police resources and investigative skills to support the protagonists.

One of Nishijima's most notable relationships is with Minene Uryuu. Despite initially being assigned to protect a target from the terrorist, he develops romantic feelings for her. He persistently asks her out for meals, much to her initial confusion and annoyance, but over time Minene comes to return his affection. He eventually proposes to her, and they plan to marry, but tragically he is shot to death before the wedding can take place. This relationship highlights his persistent, earnest, and generous nature, as well as his ability to see beyond Minene's dangerous reputation. In the final resolution of the story, a version of Minene from the second world and a version of Nishijima from the third world end up living together, providing a bittersweet happy ending for them.

Throughout the narrative, Nishijima experiences notable development. He starts as a straightforward, dutiful officer who follows orders, but as he witnesses betrayal and faces the extraordinary reality of the diary battle, he becomes more independent, proactive, and personally invested. His loyalty shifts from institutional authority to his own moral compass and the people he cares about. He does not possess a Future Diary himself, but his abilities include sharp intuition, methodical reasoning, and a strong sense of responsibility. He is also shown to be prepared for emergencies, such as carrying a firearm for self defense. His intelligence and perseverance make him a valuable asset in the investigation, even without supernatural powers.