Ms. Saijō teaches at St. Cherine Academy. In her late twenties, she grapples with acute anxieties over finances and marriage prospects. Loneliness and a dread of becoming an "old maid" propel her into multiple part-time jobs. During one job hunt, she meets the enigmatic "Mr. President"—later unmasked as the antagonist Black Trader—who hires her. She falls for him, her feelings deepening upon discovering he is a widower raising twin daughters, which further entangles her emotions.

Black Trader recruits her to help collect the Seven Amulets, pivotal magical artifacts in the series' conflict. Driven by devotion, she accepts the role of Black Carrier, despite knowing it means battling the Twin Angels. In this villainous identity, she dons a distinctive cleavage-baring bodysuit and showcases considerable combat prowess, excelling in hand-to-hand fights and deploying specialized robots. On missions, she maintains a sense of responsibility by shunning serious crimes and reducing collateral damage, often tidying destruction left in her wake. Her battle strength surges with emotional fervor, notably "boosted by the power of LOVE" during her initial clash with the Twin Angels, where she successfully claims one amulet.

Even as Black Carrier, she clings to moral boundaries, refusing heinous acts and preserving her inherent caring nature. This inner conflict sharpens as she continues teaching students like Haruka and Aoi while opposing them in her alter ego. Her double life employs a "Clark Kenting" effect: students overlook her identity despite minimal disguise beyond the bodysuit. Her journey navigates themes of desperation and moral compromise until she uncovers Black Trader's true manipulative character and his absence of authentic affection or ethics. This revelation prompts her to forsake her villainous role.