Sakie Satou is a mathematics teacher and a demi-human succubus, also called a "Dream Demon." She has fair skin, long black hair usually secured in a ponytail, and purple eyes often hidden behind red glasses. Her figure is slender yet curvaceous with a notably large bust. To reduce accidental seduction of males, she wears loose-fitting red tracksuits to obscure her shape, keeps her glasses on as a facial barrier instead of contacts, and ties her hair up. At home, she chooses simple sleepwear like tank tops and boxers without feminine designs. She lives remotely, isolated from neighbors to prevent unintended pheromone exposure, later moving temporarily to a disused school security room due to her impractical commute schedule using the first and last trains to avoid crowds. She wakes daily at 4:15 AM.

Her succubus physiology involuntarily emits potent aphrodisiac pheromones, inducing lust in nearby males and attracting gay women. The effect intensifies if she removes her glasses or wears revealing clothing, and can cause erotic dreams if she sleeps near others. These pheromones rank as the second most powerful among known succubi, with potential links to her virginity as adult cases are undocumented. Vampires experience an "Alien Catnip" reaction, becoming intoxicated by her presence. This ability causes Sakie extreme shyness, social awkwardness, and a habitual avoidance of men within a four-foot radius. She harbors deep anxiety that romantic interest stems only from her supernatural influence, recalling a high school incident where a bully subdued in a judo hold begged for more pain, exposing his masochism.

Her relationships are profoundly shaped by her condition. She develops romantic feelings for biology teacher Tetsuo Takahashi due to his apparent immunity; it is later revealed he consciously suppresses his reactions to avoid distressing her. Their relationship progresses to dating, though physical intimacy like hugging strains Takahashi significantly. After losing her virginity to him, her innate succubus traits become more pronounced despite her typically timid demeanor. She shares a love of manga with student Yuki Kusakabe, often borrowing titles. Detective Ugaki serves as her primary confidant and father figure, supporting her since age 14 after her own father struggled with her demi-human nature; he maintains cautious distance to avoid pheromone effects. Among colleagues, she admires Takahashi's self-control and prioritizes her students' well-being despite personal challenges.

Sakie's journey involves gradual self-acceptance and cautious integration. Initial isolation gives way to forming meaningful connections with Takahashi, her students, and Ugaki. She confronts her fears about relationships, taking steps to express her feelings to Takahashi despite the risks. By the conclusion of her arc, she achieves a romantic relationship with Takahashi, navigating the complexities of physical intimacy and embracing her identity as a succubus. Her path underscores overcoming loneliness, societal alienation, and pursuing normalcy despite biological constraints.