Sachiko Juraku is the main antagonist of the narrative. As Public Morals Committee Chairwoman within Hyakkaou Private Academy's Student Council, she later ascends to Student Council President through demonstrated gambling prowess, defeating even the former president. Her leadership establishes the house pet system, enforcing debt repayment via dehumanizing treatment to control the student body. Physically, Sachiko possesses knee-length light gray hair featuring a distinctive waist-length strand resting on her chest. Her narrow red eyes have cat-like pupils, and she typically wears black or dark blue nail polish. While adhering to the standard academy uniform, she substitutes a black tie for the standard female attire and wears high-heeled ankle boots instead of loafers. She is frequently seen holding a leash connected to Mikura Sado's collar. Her personality exhibits extreme sadism and a complete absence of empathy. She derives pleasure from inflicting physical and mental suffering to dismantle identities, exemplified by keeping Mikura as a house pet. Sachiko particularly relishes breaking strong-willed individuals, motivating her fixation on Mary Saotome as a potential second house pet. She prefers dominating those resisting her authority, intensifying enjoyment through their humiliation. Her formidable gambling skills include high perception of opponents' movements, reflexes, and cheat detection. Despite her cruelty, she values skilled gamblers and seeks to cultivate them. Her backstory reveals she joined the Student Council through her signature "crushing style" of gambling, defeating opponents and breaking their will. After deterring all challengers, she was appointed Public Morals Committee head. Her relationship with Mikura originated from a gambling victory, imposing continuous degradation including forced collar-wearing and public humiliation. Mikura's repeated failed revenge attempts culminated in psychological acceptance of subservience, finding enjoyment in Sachiko's domination. Sachiko encounters Mary Saotome during an investigation into Yukimi Togakushi's gambling den. Intrigued by Mary's defiance, she manipulates scenarios to test her abilities, permitting high-stakes rematches conditioned on becoming a house pet upon loss. She engineers gambles like the Coupling Party and a treasure hunt where Mary "wins" a dog collar, deliberately provoking her. Their dynamic escalates when Sachiko forces Mary into a Doubt Poker match, leveraging her ability to read micro-expressions and psychological patterns to secure victory. Though Mary temporarily evades house pet status by repaying her debt, this fuels Sachiko's obsession further. She demonstrates awareness of political maneuvering against her, including Aoi Mibuomi's plans to undermine Kirari Momobami, feigning concern while anticipating the conflict's entertainment value. She collaborates with Kirari, discussing Mary's potential submission and observing gambles involving figures like Sakura Miharutaki and Nagi Kamishimo without direct intervention.