Natsuka Kasai is an energetic high school student who previously attended junior high with Fumika Mikawa, though their first direct encounter happened in the school infirmary. She habitually pursues intellectually inclined boys, engaging with books they read to align with their interests; this pattern leads her to develop feelings for the similarly intelligent Kaname Nojima. Her role in the central story escalates when Kaname contacts her seeking information about Fumika. Prompted by this, Natsuka joins Kaname's investigation into Fumika's past, specifically the incident three years prior where Fumika shot her father, Kirameki Mikawa.

This collaboration leads Natsuka and Kaname to Kirameki's residence, where they witness a tense confrontation between Fumika, acting as the shigofumi mail carrier, and Kirameki. This event further entwines her with the unfolding truths about Fumika's dissociative identity disorder and family trauma. Physical depictions of Natsuka vary, with some sources describing brown hair and brown eyes, while others indicate gray hair and gray eyes. Her relationships center primarily on her academic connection to Fumika and her romantic interest in Kaname, with no significant development or background beyond these core interactions in the documented materials.