Mistress 9 is the Messiah of Silence, an entity who resides in Tomoe Hotaru, waiting to be fully awoken by a Pure Heart Crystal. Time to time she is able to take control of the body, which causes Hotaru to suffer from seizures. She first appears in episode 123 when the Death Busters succeed in finding the Pure Heart Crystal to awaken her. Subconsciously, Hotaru swallows Chibiusa's Pure Heart Crystal which initiated her transformation; her final awakening.
Her next stage was to obtain the Holy Grail which was in Sailor Moon's possession. She attempts to toy with other's heart by temporarily changing back to Hotaru time to time. With Sailor Moon's trusting nature, she was deceived by her eventually and successfully gains the Holy Grail. With this, the awakening of Master Pharaoh 90 begins, however; Mistress 9 is soon weakened considerably by beams which shot out from the process of his arrival. Hotaru slowly gains back consciousness within Mistress 9, and uses the power of a Sailor Senshi to eliminate her. With the aid of her memories of her loved ones, Mistress 9 is shattered and Sailor Saturn is awoken. Who would have thought Mistress 9 and Sailor Saturn both lived within her.