Mitsuki Ichijo acts as secretary to Headmaster Ryuuzouji Kouzou at Sakaimachi Academy, where she previously taught history. Her name, written with the characters for "beautiful" and "moon" (美月), carries the surname meaning "one article" (一条). Profound loneliness drives her to seek emotional fulfillment through relationships. This pattern first manifests in an affair with student Takuya Arima, initiated during shared grief—Takuya mourning his father's disappearance and Mitsuki grappling with unrequited feelings for her married college professor, Ryuuzouji. Though the relationship ends before the main narrative, they retain a connection marked by mutual warmth and concern.

Mitsuki's unresolved attachment shifts back to Ryuuzouji. His sudden behavioral changes, particularly fear toward an unnamed woman, prompt her independent investigation. Unaware that Ryuuzouji possesses hypnotic abilities, she falls victim to his manipulation, exploiting her emotional vulnerability to retrieve the Reflector Device from Takuya. Under his influence, she confronts Takuya at gunpoint on Mount Sankaku. Her affection for Takuya momentarily breaks through Ryuuzouji's control, but the internal conflict culminates in her turning the gun on herself.

This outcome is inevitable across all story routes, reinforced by the narrative's metaphor of her being "trapped in the Schwarzschild radius of the event," signifying a fixed, unavoidable fate despite Takuya's attempts to alter events with the Reflector Device. Her death demonstrates the device's limitations against predetermined outcomes.

Her original design featured a black shirt and purple suit, later revised to a light purple ensemble in the remake. Mitsuki embodies themes of emotional dependence and tragic inevitability within the story's exploration of parallel worlds and fixed points in time.