Aya Arima is the deceased younger sister of Taichirō Arima and twin sister of Asuma Arima, daughter of Kojirō Arima and Sayaka Arima. She died during elementary school before the story's main events. Her death occurred after she forgot her daily school log notebook. Kojirō, in anger, demanded she retrieve it late at night. The family alerted the school's security guard to meet her at the gate, but Aya never arrived. They discovered her murdered inside a cardboard box. Kojirō publicly claimed her death resulted from a car accident, though the family knew the truth, causing lasting trauma and resentment that fractured the family.

Aya possessed dark purple hair styled to cover one eye, purple-tinged eyes, and often wore a small flower in her hair. She is frequently depicted in a white shirt and pink apron.

Her death profoundly shaped the family's dynamics. Kojirō's grief drove him to create the virtual reality game "Planet," modeling its final boss, the "Black Bird of Happiness," on crows likely present near her body. This entity evolved into a dangerous AI capable of harming players in reality. The unresolved trauma fueled the family's reliance on Planet as an escape.

Later, the Black Bird entity merged with Aya's lingering presence or memory, manifesting in both the game and reality. It gained the power to warp perceptions and physically manifest, triggering real-world chaos. This connection culminated in Kojirō, the merged Black Bird/Aya entity, and his assistant vanishing during a confrontation.

Aya's absence continues to haunt the survivors. Sayaka lives as a vagrant, consumed by grief. During a pilgrimage by Taichirō and Asuma to a location tied to family memories and Planet, a momentary glitch suggests Aya's unresolved influence or the blurred boundaries between reality and the game's legacy.