When Tomoe Hotaru was a small child, a fire accident killed her mother Keiko and left Hotaru very injured. Her father, Tomoe Souichi, was able to keep her alive by partially rebuilding her body. However, he was unable to return her to full health, and as a result she is very weak and subject to fits of pain. Despite this, she seems to be very strong when she wants to be, and she is able to heal with a touch.
Professor Tomoe is very rich (he owns the Mugen Gakuen school), and Hotaru lives in luxury even though she is physically frail. It turns out that Tomoe Souichi, her father, had begun creating demons in his laboratory, and was working with Kaolinite and the Witches 5 to help the evil Pharoah 90 gain access to this world. He was also nurturing the form of Mistress 9 inside the weak body of his daughter -- Mistress 9 being the one who can open the doorway for Pharoah 90.
Hotaru-chan's black/dark purple hair is cut short, and she has large gray eyes. She likes to wear dark colors and concealing clothing, because there are still scars left on her body from the accident. Although she is pretty, she is lonely; she is solemn and shy and her physical weakness sets her apart from her peers.
However, one day a cute, enthusiastic pink-haired sailor senshi saved her from a demon; she introduced herself as Chibiusa and seemed eager to make friends. Chibiusa felt sorry for Hotaru, and wanted to help her get stronger; Hotaru was not about to drive away the only friend she had. However, the evil grew uncontrollably inside her, and eventually Mistress 9 took over Hotaru's body, strengthening it with dark energy... and stole the pure heart of Chibiusa. The pink-haired little senshi would have died, had not Mamoru's healing energy kept her alive. But I digress.
Hotaru's spirit lived on, and fought Mistress 9's possession of her body. Eventually, she gained control, and restored Chibiusa's heart. However, all this fuss had awakened the other spirit sleeping inside her -- Sailorsaturn, senshi of destruction.
You see, this is a bad thing. Sailorsaturn is the senshi of destruction, she who sleeps until the end of an age. Then she awakens, and raises her Silence Glaive. And when it falls, its blade cutting the air, it brings about the end of the world. The last time she awoke was at the end of the Silver Millenium; the fall of her Silence Glaive was what ended that era. And now that she was awake, this era also had to come to an end.
She lowers the Glaive, and the city explodes... but out of the destruction rises Sailormoon, the senshi of rebirth. Sailorsaturn follows Pharoah 90 through the gateway to the other world, and disappears as Sailorpluto closes the door behind her. Sailormoon uses the Illusion Silver Crystal to heal the world - and in the ruins of the battleground, they find a dark-haired baby with the sigil of Saturn on her forehead.
Haruka, Michiru, and Setsuna take her, vowing to raise her themselves, that this time she might have a chance at a happy childhood and a normal life. (Though how normal that life would be, being raised by three senshi, I hesitate to guess...) It is obvious, however, that little Hotaru is much happier with her new family. And when she grows to be a healthy, kawaii little girl, she grows into her powers once more -- properly this time. As Sailorsaturn, she is the senshi of ruin and birth. Her attack: "Death Reborn Revolution." She represents silence and destruction, counterpoint to Sailormoon.
Hotaru is a frail little character, but she possesses an iron will. As a young child, she fought off fits of pain and became a sweet little girl, despite the fact that her father was a mad scientist who tended to ignore his daughter's feelings. When her body was possessed by Mistress 9, she fought the evil spirit unceasingly until she was free again. And when she was free, her first thought was for the well-being of her friend Chibiusa. When she was reborn into her new life, she and Chibiusa were still the best of friends.
Credit goes to Sexylyon, without whom I would never have been able to figure out exactly what went on in manga volume 9. (She's got manga translations, bless her...) And her relation of the events that transpired is much clearer than mine. And many thanks to Ian Andreas Miller, who gave me the correct translation of Saturn's senshi title, as well as other great pieces of information.