Ichinose Reiko is the mother of protagonist Kai Ichinose. She lives with her son in a dilapidated neighborhood bordering a forest, the same forest separating her from her own birthplace. Her circumstances led her to work as a bar hostess and prostitute. She became pregnant at 14 and gave birth to Kai at 15, making her 25 at the story's start. Her youthful appearance frequently leads others to mistake her for Kai's sister.

Reiko harbors a deep affection for the ocean, though she has never seen it. This longing inspired her to name her son "Kai," meaning "sea." Her profession attracts significant social stigma within the community. Residents often mock her as irresponsible and immoral, partly due to the unclear paternity of her child. Rumors suggest Ben-chan, a local handyman roughly ten years older than Kai, might be the father, though this remains unconfirmed. Despite societal judgment, Reiko openly expresses disdain for her work and distrust toward men.

Her relationship with Kai is marked by mutual closeness and devotion. She prioritizes his well-being and supports his musical pursuits. Recognizing Sosuke Ajino's genuine admiration for Kai's piano talent and his refusal to engage in paternity speculation, Reiko develops profound trust in him. She feels secure entrusting Kai to Ajino's mentorship, viewing him as a stabilizing influence.

Later in the narrative, Reiko remains connected to Kai's journey. She attends a significant piano recital featuring both Kai and Ajino, accompanied by friends from the Forest's Edge community. Her appearance demonstrates her ongoing support for her son's career and acknowledgment of Ajino's role.