Mistress 9 is an extraterrestrial entity from the Tau Star System, a parallel dimension also known as the Tau Ceti Star System. She serves as the primary partner and subordinate of the cosmic being Pharaoh 90. Her core objective is facilitating Pharaoh 90's arrival on Earth to merge with and transform the planet, requiring a human host vessel. This leads to her implantation into Hotaru Tomoe, the young daughter of genetic scientist Professor Souichi Tomoe. Professor Tomoe, conducting unethical experiments to create superior life forms, willingly collaborates with the Death Busters organization after encountering them during a lightning storm. He implants Mistress 9's Daimon egg into Hotaru's body following a laboratory accident that killed Hotaru's mother and left Hotaru severely injured. This implantation causes Hotaru to suffer chronic, debilitating seizures throughout her childhood, alleviated only by a fragment of the Taioron Crystal or later by exposure to the Silver Crystal's energy. Upon taking control of Hotaru's body, Mistress 9 radically alters its physical form, transforming the adolescent body into that of a tall adult woman with extremely long, straight black hair extending past her feet. Her eyes are typically violet but shift to red during moments of heightened power or emotion. She wears a distinct dark purple or violet floor-length dress adorned with strings of pearls, a golden headdress resembling a Russian kokoshnik, a golden leaf choker and belt, and Galactica Bracelets. Her makeup includes orange or purple lipstick and light purple eyeshadow, and she wears dangling three-pearled earrings. This appearance bears a noted resemblance to Hotaru's late mother, Keiko. When forced out of Hotaru's body or manifesting independently, Mistress 9 assumes her true extraterrestrial form: a large, dark purple, muscular creature covered in spikes with white eyes. During her merger with Pharaoh 90, they manifest as a massive liquid entity with red eyes. Mistress 9's awakening is gradual, fueled by proximity to the Silver Crystal carried by Chibiusa. Sensing its immense power, her influence over Hotaru intensifies, leading to periods where she seizes control, causing blackouts and violent episodes. She fully awakens upon physically taking Chibiusa's Prism Heart Compact, extracting the Silver Crystal, and consuming both the crystal and Chibiusa's soul. This act renders Chibiusa comatose and allows Mistress 9 to fully manifest within Hotaru's transformed adult body. Immediately upon awakening, she dismisses and kills the Death Busters operative Kaolinite, deeming her services unnecessary. Mistress 9 then fortifies Mugen Academy, the Death Busters' headquarters, with protective vines adorned with thorns and black roses. Despite Mistress 9's control, Hotaru's consciousness persistently resists from within, actively working to thwart her plans and protect Chibiusa's soul and the Silver Crystal from full assimilation by Pharaoh 90. This internal conflict manifests physically through debilitating migraines or paralysis in Mistress 9. Mistress 9 expresses disdain for Hotaru's body, finding it confining and the long hair cumbersome. Her primary goal is using the Silver Crystal's power to open a stable gateway to the Tau Star System, enabling Pharaoh 90's full manifestation on Earth. She communicates directly with Pharaoh 90, presenting him with the crystal and later swallowing it to amplify her own power for the gateway ritual. To further empower herself and overcome Hotaru's resistance, she attempts to consume the souls of the four Inner Sailor Guardians. Hotaru's spirit intervenes, retrieves the guardians' souls along with Chibiusa's, and escapes Mistress 9's body to return them. This act weakens Mistress 9 significantly and damages Hotaru's physical body. Forced out of the compromised human vessel, Mistress 9 manifests in her true monstrous Daimon form, engaging in direct combat with the Sailor Guardians. She demonstrates high resilience, absorbing or enduring powerful attacks, including combined assaults from Super Sailor Moon and Super Sailor Chibi Moon. To achieve her ultimate goal, Mistress 9 merges with the manifesting Pharaoh 90. This merged entity successfully breaches the dimensional barrier, opening the portal to the Tau Star System and beginning the process of engulfing Earth. However, this act triggers the awakening of Sailor Saturn's power from within the remnants of Hotaru's being. Sailor Saturn fatally wounds the merged entity using her Silence Glaive and the "Death Reborn Revolution" attack. Mistress 9 perishes alongside Pharaoh 90 as they are forcibly expelled back into the Tau Star System, with the dimensional gateway permanently sealed behind them by Sailor Pluto. Mistress 9 possesses several abilities. She manipulates and extends her long black hair like prehensile tentacles to ensnare, strangle, or attack opponents. She exhibits the power to extract and consume souls. She summons and controls thick, defensive vines studded with thorns and black roses. After swallowing the Silver Crystal, her power increases substantially. In her true monstrous form, she projects crystalline spikes from her body and elongates her limbs for physical attacks.