Koito Nagase, a transfer student with chest-length brown hair, emerald-green eyes, and glasses, became the sole friend of the protagonist during their shared isolation. Her navy school uniform, dark green tie, and white socks underscored her composed demeanor, reflecting her calm, kind, and mature nature. Despite enduring bullying she dismissed as jealousy over her newcomer status, Koito sustained an unwavering optimism.

Their bond began when Koito followed the protagonist home, her persistent curiosity breaking through the protagonist’s solitude. Her relationship with homeroom teacher Sawaki carried undercurrents of ambiguity, punctuated by a witnessed embrace that contradicted Koito’s reserved explanations. Her death unfolded during a confrontation on the school roof, where she threatened to expose Sawaki for misconduct before an accidental fall—initially mistaken for suicide—plunging the protagonist into guilt-driven isolation and a desperate quest for supernatural resurrection.

Though Koito’s friendship initially appeared selfless, later reflections hinted her kindness might have been rooted in empathy for the protagonist’s own bullied existence rather than pure affection. Her erratic actions—false accusations, the rooftop confrontation—hinted at unspoken psychological turmoil. Her legacy, symbolized by her name’s meaning ("small thread"), weaves through the protagonist’s journey of guilt and unresolved grief, driving a search for closure. Fragmented memories and retrospective accounts sketch her background, leaving her family, history, and true motivations deliberately obscured in official narratives.