Mistress 9 is a Daimon from Tau Nebula; an evil spirit who serves Pharaoh 90. She is an evil female entity from "Tau 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).
After taking control of Hotaru's body, she ages it up to that of a 30-year-old with incredibly long dark hair, purple eyes, and dark purple lipstick. She wears a blue sleeveless top with white beads and a long white dress. She also wears earrings with white beads.
After his daughter Hotaru was badly injured in a lab incident, Souichi Tomoe was about to repair her body with his assistant Kaori when they were approached by a being from another world, Pharaoh 90, who transformed Kaori into the Magus Kaorinite and gave Souichi the Daimon egg that contained his partner, Mistress 9. He told him to find her a host, promising great power and knowledge if he did so.
Souichi then implanted Mistress 9's Daimon egg within his daughter, leading to Hotaru's frail physical state, causing her to suffer from seizures that are soothed by what Souichi claims to be amulet occasionally be taken over by Mistress 9.
The Death Busters are frequently striving to awaken Mistress 9 deep within her host, Hotaru. When Hotaru meets Chibiusa and the latter shows her the Silver Crystal, Mistress 9 becomes aware of its power and later steals it from Chibiusa (along with her soul), 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 Sailor Senshi, her plan begins. Mugen Academy itself has ripped apart, throwing the unconscious bodies of Mercury, Mars, Jupiter, and Venus 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, only for Hotaru to retrieve them and escape her body with them and Chibiusa's soul to return them to their respective 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 and Hotaru's spirit form fades away.
She absorbs all of the Sailor 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 Nakayoshi Character Poll for the Infinity arc, Mistress 9 was listed at the 23rd most popular character.
- It is interesting of note that when Mistress 9 aged Hotaru's body to that of an adult woman after gaining control over her, she bore a striking resemblance to her mother, Keiko. As it was Hotaru's actual body and not an assumed Daimon form of Mistress 9, this indicates that Hotaru will grow to strongly resemble her mother when she reaches adulthood.