Moe Nishinosono is a first-year student in the architecture department of Nagano University and the female lead of the story. She comes from a wealthy, high-class family and is the daughter of Souhei Saikawa's former mentor, a connection that gives her a natural entry point into the investigative world she becomes drawn into. Her background is marked by a deep personal tragedy: her parents died in a plane crash, an event that profoundly shapes her perspective on life and death and informs her emotional responses throughout the narrative. She is highly intelligent, possessing sharp insight, keen observational skills, and a particular talent for rapid mental calculations, though her enthusiasm sometimes leads her to jump to extreme conclusions. Her personality is defined by strong intellectual curiosity and emotional openness; she is far more emotionally driven than the reserved and analytical Saikawa, and she is not afraid to voice her questions or discomfort directly. This contrasts with Saikawa's detached logic and makes her an effective conduit for the audience's perspective. She loves mystery novels and the sound of car engines, and she has a peculiar habit of eating dried shiitake mushrooms every day despite disliking them, because she believes they are good for anemia.

Her central motivation is her deep affection for Souhei Saikawa, whom she actively pursues romantically. She frequently uses her family's resources to initiate trips that lead them into mysteries, such as the visit to the isolated Himaka Island research facility. In the story, she serves as both a partner and a foil to Saikawa. She views the reclusive genius programmer Shiki Magata as a rival for Saikawa's intellectual attention and as a disturbing figure of mystery. Her role evolves from that of a student seeking Saikawa's attention and approval into a capable investigator in her own right. As the investigation unfolds, she is forced to confront her past trauma through the lens of the Magata case, gaining a more mature and nuanced understanding of Saikawa's personality and human relationships. This development allows her to grow beyond her initial infatuation and become a more independent and insightful participant in solving the mystery. Her relationship with Saikawa is a complex mentor-student bond where she often handles his social interactions, and her interactions with other characters, such as the mature hostess Setsuko Gidou, highlight her insecurity and one-sided rivalry, though she ultimately matures through these experiences. Her notable abilities lie in her rapid mental calculations, her powers of observation, and her ability to ask the right questions that push the investigation forward, compensating for her lack of formal expertise with sheer curiosity and emotional intelligence.