Hiroko Takashiro cultivated a fascination with witchcraft during her academy years alongside classmates Rie and Saki. Discovering the Bible Black spell book, she founded the Rose Cross club. She first tested its power by casting a lust spell on student council member Junko Mochida, causing a public scandal. The club amassed influence and monetary success selling love charms, including one used on student council president Nami Kozono. Driven by ambition, Takashiro attempted a demonic summoning; the entity raped her, leaving her comatose for three days. This trauma led her to reject dark magic. Her friends proceeded with a Walpurgis Night ritual without her, resulting in their deaths and the book's disappearance.

Twelve years later, she returned to the academy as an art teacher and faculty advisor, concealing her occult knowledge while studying white magic. Despite her popularity, psychological scars resurfaced as sexual flashbacks, culminating in an encounter with the possessed vice-principal. When occult disturbances recurred, she grew suspicious of school nurse Reika Kitami and warned art student Taki Minase, though he dismissed her. Kitami, seeking revenge for her role as the intended sacrifice in the failed ritual years prior, had Takashiro kidnapped. She was subjected to water torture and rape. Drugged with sexual stimulants, Takashiro was later forced into sex with several art students. After escaping with Minase's help, she discovered the Bible Black in his possession and learned of Kitami's devilish contract. Together, they researched a forbidden spell to stop Kitami from transferring her soul into student Kurumi Imari. During Walpurgis Night, Takashiro breached the ritual's barrier, neutralized obstacles including Kaori Saeki and her followers, and assisted Minase in disrupting the ceremony, causing Kitami's physical demise.

Following these events, Takashiro erased the memories of those involved but later discovered Kitami's soul persisted within Imari's body. She bound the soul to stifle its influence, causing Imari to forget. Seeking permanent solutions, she abandoned teaching to study religious history and pursue purity, adopting life as a nun. Her journey involved confronting bodily temptations, surviving a sexual assault by two thugs, and encountering a monk at a monastery where an exchange involving sex occurred for an ancient scroll. In the New Testament timeline, she collaborated with Toru Yuge of the Tokken psychic agency against the revived Kitami (controlling Imari) and occultist Jody Crowley. During this conflict, Kitami overpowered and raped her in a greenhouse. Takashiro persisted in efforts to thwart Crowley's ritual involving the Spear of Longinus and the "Scarlet Woman," Aki Ichikawa.