Rio Kurotori, a master magical toolmaker and ex-instructor at the Magical Law School, crafts powerful artifacts others wield despite her inability to use them personally. Her workshop, "Rio’s Office," specializes in tailoring tools by harmonizing her spiritual energy with a client’s to amplify their efficacy—a skill that produced Toru Muhyo’s iconic magical lawbook.

Her past is haunted by her mother’s terminal illness, which drove Rio to desperate measures. Exploited by a practitioner who threatened financial ruin, she endured coerced humiliations, including wearing revealing attire. When a spirit attack killed her mother and enforcers denied aid, her fury at the Magical Law Association solidified. Grief-stricken and betrayed, she allied with Soratsugu Madoka (Enchu), embracing forbidden soul-draining magic that corrupts users into vengeful spirits.

Initially dismissive of her apprentice Yu Abiko (Biko), Rio’s disdain turned to devotion after Biko rescued her from a botched experiment. Though she later betrayed Biko for Enchu, Rio clung to hopes of redemption. Discovering her mother’s death was orchestrated by Teeki spurred her to confront her crimes. Accepting accountability, she entered house arrest with Biko, pursuing reconciliation.

Rio balances flirtatious charm—earning the moniker "kissing fiend"—with vulnerabilities: a penchant for Biko’s homemade bread and coffee, and a low alcohol tolerance. Her inventions, like the Incinerator’s Cane, Fireball, life-restoring Ren Source Spring (lethal in excess), and Physical Barrier Spells, underscore her technical brilliance despite her inability to activate them.

Her story culminates in restrained mentorship under supervision, reflecting a transition from vengeance to atonement as she continues crafting tools within the confines of her reformed life.