Mistress 9 is an evil female entity from "Tau Ceti Star System" who has come to Earth to help open a gateway for her master, Pharaoh 90. She possesses Hotaru Tomoe's body (to varying degrees of intensity).
She has long black hair and dark eyes. She wears a blue sleeveless top with white beads and a long white dress. She also wears earrings with white beads.
The Death Busters are frequently striving to awaken Mistress Nine deep within her host, Hotaru. When the Death Busters first visited Professor Tomoe (as he was reconstructing his fifteen-year-old daughter Hotaru's body), one of the objects they gave him was an egg that contained Mistress 9. He implanted it within Hotaru's body, but the consequence of this action was Hotaru's permanent frail physical state. She occasionally suffers from seizures that are soothed at first by an amulet given to her by Tomoe (he tells her it is an heirloom, but it is more likely a fragment of the Tau Crystal) and later by the Silver Crystal. When Chibiusa shows it to Hotaru, Mistress 9 becomes aware of it and later grabs it from Chibiusa, sending her into a comatose state.
When Mistress 9 overpowers Hotaru's mind and body, it becomes that of a grown woman. Her facial features are maniacal and there is little resemblance to the girl whose body she has taken over. Her purpose is to create an opening to the Tau Ceti Star System so that Pharaoh 90 can access and assimilate himself into Earth.
Once all her allies have been killed and is confronted by Sailor Moon and the Outer Senshi, her plan begins. Mugen Gakuen itself is ripped apart, throwing the unconscious bodies of Mercury, Mars, Venus and Jupiter into the air (they come to rest on a structure that Kaorinite previously used for divination). She then attempts to escape Hotaru's body (meaning that her power will be able to flow freely) but is surprised to find that the soul of Hotaru still exists within the body and is inhibiting her from leaving it. Nonetheless, she swallows the souls of the Guardian Senshi where upon Hotaru retrieves their Heart Crystals along with that of Chibiusa and escapes to return them to their owners. While she is doing this, Mistress 9 is able to leave Hotaru's body, ripping it apart in the process. Her true form is of a gigantic monster with a ghoulish head and an indistinct body.
She absorbs all of the Senshi's attacks and opens the door for Pharaoh 90, who begins to smother the planet. Mistress 9 is assimilated into the lava (which presumably is Pharaoh 90) and just when all seems lost, the spirit of Sailor Saturn herself rises from it. When she sets her Silence Glaive down into the ground on its sharp sickle point, both Pharaoh 90 and Mistress 9 experience intensely vast agony, weakness, and paralysis. To finish the job once and for all, Sailor Saturn uses her immensely powerful and destructive Death Reborn Revolution attack to drain the last of their remaining strength and they both rise into the sky to escape back to Tau System. Sailor Saturn follows them and commands Sailor Pluto to seal the door for eternity. Pluto does this and assures that Mistress 9 and Pharaoh 90 can never reach Earth again.
- In the anime, Mistress 9's human appearance is her true form.
- In the Nakayoshi Character Poll for the Infinity arc, Mistress 9 was listed at the 23rd most popular character.