About Hotaru
Forgive me if my info is not 100% accurate, it's been awhile since I watched the S series, and my memory is not too fresh on it. If you find anything inaccurate or anything I may have forgotten, please don't hesitate to email me. Thanks. Now on with the info. Hotaru shows up in the S series, at first she is shown as Professor Tomoe's daughter and she is very weak and gets sick a lot. When she was little there was an accident in her fathers lab one time, and if her dad let his body be posessed, along with Hotaru they'd be allowed to live. Of course he didn't want Hotaru to die, so he allowed this. She also has the power to heal which scares her classmates. BUT she has another side to her, Mistress 9, who is trying to help Pharoh 90 and take over the world, lives inside her body. To keep Mistress 9/Hotaru alive, she must have heart crystals. Her father ,whose body is also used by evil beings, works in a lab in the basement of their house and with the help of the Witches 5, he tries to create daemons which will capture heart crystals from those with pure hearts.
One day Chibi-usa is at the park and her hat gets blown off by the wind, when she goes chasing after it, she meets Hotaru, who had saved her hat. From then on they become good friends. Eventually the senshi find that Hotaru has strange powers and they believer she is evil, so they try to keep Chibi-usa away from her. The reason Hotaru/Mistress 9 must be kept alive is because she is the so called "Messiah Of Silence" who can take over the world and seal it with darkness, if she gets the sacred cup.
Eventually it is revealed the Usagi/Sailor Moon is the true Messiah, and since she is good, the world will be filled with peace, instead of destruction, like it would if the Messiah of Silence had it. Soon Hotaru was shown to be Sailor Saturn, but they still wanted to kill her for being evil. Chibi-usa of course wouldn't let them and Usagi defended her also. At the end of the S series, Sailor Saturn goes into the explosion and destroys it with her glaive, she gets destroyed along in it. Then she gets reborn as a little baby again and doesn't come back till Sailor Stars.