Kaori Saeki attends St. Walpurga Academy, distinguished by her profound fascination with the occult and extensive knowledge of black magic. Her long blond hair is typically styled in a tall ponytail with side locks, framing distinctive violet eyes. She wears the standard school uniform, a black robe for occult club activities, and a swim cap at the academy pool. Outside school, she favors a sleeveless white top, black miniskirt, and thigh-high stockings, accessorized with silver crescent-shaped earrings.

As founder and president of the school's occult club, Saeki actively researches supernatural phenomena and conducts tarot card fortune-telling sessions for classmates. Her obsession leads her to discover classmate Taki Minase's possession of the "Bible Black" grimoire; she subsequently attempts to seduce him, offering oral sex in exchange for information about the book. This encounter reveals her willingness to use her body and manipulative skills to advance her occult interests.

Her involvement intensifies when school nurse Reika Kitami sexually subdues Saeki and her club members Jun and Maki through violent rape and mind control. This trauma, combined with her existing thirst for occult power, transforms her into Kitami's loyal subordinate. Under Kitami's influence, she participates in kidnapping, torturing, and sexually abusing fellow students and faculty to further Kitami's ritualistic goals. Her compliance stems from both fear of punishment and eagerness to learn authentic black magic.

Years later, Saeki returns to the academy as a teacher, now clad in professional attire: a grey undershirt, lavender miniskirt, brown high heels, and a long white coat. Her occult obsession remains undiminished; she reforms the witchcraft club with new students and seeks to restore the Bible Black's power. She again falls under Kitami's control, resuming violent orders including torturing and raping enemies. However, following Kitami's defeat, Saeki undergoes significant development: expressing remorse for her past actions, she renounces dark pursuits and becomes a Makai Priestess dedicated to protecting the innocent, signaling a path of atonement.

Throughout her arc, she demonstrates proficiency in spellcasting and exhibits partial resistance to enemy magic, though these abilities are inconsistently portrayed across different media. She resides in a modern home near the academy; her room is filled with occult literature and paraphernalia, reflecting her lifelong dedication to esoteric practices.