Rie Matoba, sister to Shin Matoba, belongs to the affluent Matoba family. A Grimm attack in their childhood inflicted severe injuries on both siblings and claimed their mother's life. While Shin recovered, Rie endured long-term hospitalization due to the severity of her wounds.

During her confinement, Dr. Kagatsuka oversaw her care, subjecting Rie and other patients to unethical experiments. Manipulated and tortured under his influence, she was forced to kill friends within the hospital walls, an experience that shattered her psychologically.

Consumed by vengeance against Kagatsuka for his cruelty and manipulation, Rie confronted him. The intensity of her emotions during this encounter triggered her transformation into a Busoma—an armored monstrous entity tied to cursed weapons. In this monstrous state, she impaled and killed Kagatsuka as retribution.

After transforming, Rie encountered Marcus Lithos, fully possessed by the demonic sword Azoth and also a Busoma. Azoth/Marcus then ripped off her face, demanding to see her "true self." Her corpse was subsequently used as a shield in the ensuing conflict.

Her brother Shin remained painfully aware of her suffering during her hospitalization, citing it as part of his own trauma. His protective feelings towards Rie fueled his hostility towards Gai Ogata, whom he perceived as a threat.