Taki Minase, a high school senior and art club vice-president at an unnamed academy, resides with his cousin Yukiko Minase—his legal guardian since his parents' work-related relocation. Their bond mirrors siblings despite no blood relation. His lifelong friend and neighbor Kurumi Imari, art club president and classmate, wakes him daily per his parents' request. Their shared childhood included brief Shorinji Kempo training, which he abandoned.

Physically, Minase sports spiky brown hair, green eyes, and a moderately tall, slim frame. He predominantly wears his school uniform, occasionally switching to a black robe for magical endeavors or a rugged jacket in select situations. Lazy, mischievous, and selfish by nature, his latent perverse tendencies escalate after uncovering occult power. Though artistically gifted—proven by a realistic portrait of Imari—he initially dodges art club duties.

Minase's descent begins upon discovering the "Bible Black" grimoire in a sealed basement. Using dictionaries and Yukiko’s linguistic aid, he translates spells. Testing a paper-doll love charm on student council president Rika Shiraki, he ignites a sexual affair. Repeating the charm for classmate Ayumi Murai confirms the magic’s power, attracting occult enthusiast Kaori Saeki, who seduces him for book knowledge. School nurse Reika Kitami confronts him in the basement; after sexual contact, she implants the lesser demon Rasha into him, seizing psychic control.

Under Kitami’s command, Minase corrupts Saeki’s occult group and captures art teacher Hiroko Takashiro—a survivor of prior witchcraft. He joins Takashiro’s torture and witnesses Kitami’s orchestrated sexual assaults, including Shiraki’s. When Imari interrupts an art club orgy, Minase tries magically stunning her. Imari’s slap shatters Kitami’s hold, freeing him. He rescues Takashiro, sheltering her at home. Kitami retaliates by magically forcing Minase into non-consensual sex with Imari, then compels him to watch her assault by others.

Seeking redemption, Minase allies with Takashiro, who reveals Kitami’s plan to soul-transfer into Imari’s body. They translate a blood-stained "Forbidden Spell" from the grimoire. During Kitami’s Walpurgis Night ritual, Minase infiltrates the basement robed. Facing illusions of Yukiko and Shiraki being raped, he chants the spell, halting the ritual and preserving Imari’s soul. Surface normality resumes; Minase and Imari become a couple. Unbeknownst to him, Kitami’s soul persists within Imari.

Minase later vanishes inexplicably. Sequel media implies Kitami (in Imari’s body) resents Takashiro for "stealing" him, hinting at his absence or death.