She wears her black hair straight, and loose down her back, and has bangs black eyes and wears a long black 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 Karinite 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.