In the manga, Mistress 9 awakened in Hotaru years after the latter nearly died in an accident and her father implanted a Daimon Egg into her. Immediately after awakening, Mistress 9 stole Chibiusa's Silver Crystal, nearly killing her. Mistress 9 then tried to drain power from the Crystal for Pharaoh 90, but Hotaru's spirit blocked her from doing so. In need of power, Mistress 9 tried to eat the souls of the four Inner Senshi, but Hotaru prevented this as well.
As her battle with the Senshi progressed, Mistress 9 grew powerful enough to leave Hotaru's body, and became a giant monstrous form that no attack could hurt. Sailor Moon destroyed her by entering the monster and releasing her power from the inside. This caused Mistress 9 to explode, releasing Sailor Saturn.
Mistress 9 was shown to dislike Hotaru's body, considering it confining and the hair annoying.
In the anime, Mistress 9, after taking control of Hotaru's body, was actually able to convince Sailor Moon to give her the Holy Grail. She destroyed the Grail, thus rendering Sailor Moon unable to transform into Super Sailor Moon. However, the power of Sailor Saturn awakened within Hotaru and destroyed Mistress 9. Sailor Saturn appeared in spirit form to return Chibiusa's Pure Heart Crystal, then sacrificed herself to go deep into the core of Pharaoh 90 to destroy it. Sailor Moon refused to let Hotaru die, and, using the strength of her own Pure Heart Crystal, became Super Sailor Moon and saved her.