Mistress 9, also known as the Messiah of Silence, was an evil entity from the Tau Star System who was implanted into Hotaru Tomoe's body.
While not fully awakened, she simply took control of Hotaru Tomoe's adolescent body. She appeared to wear a purple long-sleeved top with an outer frill along the arms, and a very long ruffled light lavender skirt. The top was topped with a lavender collar in the same shade and design as the skirt, and had black lines meeting at a hole near the neck of the shirt, where a black star (the symbol for the Death Busters) was visible. Most notably, there appeared to be a rather oversized red bow on the front of the waist of her dress. In her fully awakened form, she wore her very long black hair straight and loose down her back, with black eyes wearing a long blue dress with bead straps.
After being possessed by Germatoid, Professor Tomoe chose his daughter, Hotaru Tomoe, to be the host of Mistress 9. She needed Pure Heart Crystals to increase her power, which is why Professor Tomoe assigns the Kaorinite and the Witches 5 to the task of finding Pure Heart Crystals.
While Mistress 9's power was limited in Hotaru's body, there were times were she would cause her host to blackout and take over her body, making her commit violent acts without her knowing.
Sailor Moon S
Kaolinite and the members of Witches 5 continued seeking out Pure Hearts at Professor Tomoe's instruction. Mistress 9 ordered him to collect more Pure Hearts after the Holy Grail was obtained by Sailor Moon, which Tomoe first interpreted as collecting valuable hearts (by Mimete's standards) and then focused shortly on collecting large quantities of them. As her power grew, she gained more control over Hotaru's body, which culminated when Kaolinite helped Mistress 9 absorb Chibiusa's Pure Heart. This caused the true awakening of Mistress 9, causing Hotaru's body to transform by growing older, letting her hair grow out extremely long, and her outfit changing into a sleek black and blue dress. Saying that Kaolinite served her purpose and is not needed anymore, Mistress 9 killed her with a bolt of electricity. Sailor Uranus and Sailor Neptune attacked Mistress 9, but Sailor Moon jumped in front of her and took the attack to protect Hotaru. Mistress 9 threatened to kill Sailor Uranus and Sailor Neptune unless Sailor Moon gave her the Holy Grail.
Professor Tomoe, now free of Germatoid's control, appeared, and Hotaru appeared to regain control and ask for the Holy Grail to return to normal. However, Sailors' Uranus and Neptune sensed a trap set up by Mistress 9 and warned Sailor Moon not to give it away. However, sensing Hotaru was still in there, she and Professor Tomoe gave her the Grail. The trick worked and she used it to summon Pharaoh 90, destroying the Holy Grail in the process. However, she was attacked by Pharaoh 90 but was protected by Sailor Moon and Professor Tomoe. Hotaru ultimately overpowered Mistress 9 with strong memories of Chibiusa and Tomoe and awakening the spirit of Sailor Saturn herself deep within Hotaru, and she was destroyed.
- In the manga, her true form was similar to Pharaoh 90 and was only trapped in Hotaru's body and contorted due to her appearance, whereas in the anime, Mistress 9's form is her true form.
- She is the only villain to awaken slowly over the course of multiple episodes.