Appearance: Episode 123
Seiyuu: Minaguchi Yuko (I am assuming that Mistress9 and Hotaru have the same Seiyuu.)
Mistress 9 is the evil persona that is being housed in the body of Hotaru Tomoe.
In this season, Tomoe Hotaru is introduced for the first time. She is indeed, by far, the most complicated and mysterious of all the main characters. She is first introduced as a weak child with not many friends. She often collapses and goes into weird trances.
In her past, she was killed in an explosion in her father's lab. Professor Tomoe pleaded for her not to die and pleaded for help. He was then possessed by a Daimon and Hotaru became the shell for Mistress 9. This was the agreement that let Hotaru live.
She and Chibi Usa become very quick friends. Hotaru, having no knowledge of housing Mistress 9, is afraid that she will hurt Chibi Usa, but is delighted to have a real friend. She also shows Chibi Usa her power in healing when Chibi Usa is injured.
As an alter ego, Hotaru is seen as a girl in a chair hidden in shadows talking in a deep evil voice demanding pure heart crystals and the Holy Chalice.
When Mimete attacks the Inner Senshi at the planetarium, the saturn symbol flames on her forehead and everybody can hardly believe that she is a Senshi as well. The Outer Senshi try to kill her knowing that she is the Soldier of Destruction. Chibi Usa and Sailor Moon stop them and the girl disappears. Hotaru remembers nothing from this incident and goes to find Chibi Usa later being given one of Tellu's evil plants (Hotaru does't know it's evil). While she's with Chibi Usa, Hotaru starts to become possessed by Mistress 9. She becomes frightened and pleads for help with Chibi Usa and Usagi. Once she overcomes the possession, Kaolinite takes her away.
When Chibi Usa goes to save Hotaru herself, Kaolinite kidnapps her. Hotaru then steals and swallows Chibi Usa's pure heart cyrstal she now becomes Mistress 9. She is now the Dark Messiah with the job to prepare for the coming of Master Pharoah 90. She also becoems responsible for killing Kaolinite, who no longer was needed.
She demains the chalice from Sailor Moon and even tries to trick her into giving it. Neptune and Uranus try to kill Mistress 9, but Sailor Moon will not let them. She then successfully tricks Sailor Moon into giving it to her by saying that Hotaru needs it to survive: Sailor Moon's goal (to let Hotaru live). Master Pharoah 90 then destroys the Chalice for it's unlimited powers. Through pleadings of the now good Tomoe and Sailor Moon, Mistress 9 is destroyed by the power of Sailor Saturn and the prevalence of Hotaru's true self. As Sailor Saturn, she returns Chibi Usa's pure heart and then returns to the scene confronting and thanking Sailor Moon for saving her body of Saturn.
Saturn tells that she is the sacrificing Senshi and must go in to defeat Pharoah 90 to save the world. She holds her Glaive out to stop Sailor Moon and backflips into the heart of the destruction. Sailor Moon finds her power and goes in after her. Once the dust settles, Sailor Moon comes out with a now baby Hotaru. Neptune and Uranus take the baby from Sailor Moon and raise her for a few days. They then give the baby back to the healed Professor Tomoe.