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In the beginning she was Sailor Saturn, one of the four Outer Guardians of the Silver Millenium. Like the other Sailor Senshi she fell when the Moon Kingdom was attacked by Queen Metallia, but her soul was saved by Queen Serenity and sent to Earth where it could be reborn.
Saturn was reborn as Hotaru Tomoe, the daughter of scientist Souichi Tomoe and his wife, Keiko. Keiko died when Hotaru was a very small girl, so it was just her and her father at home. When she was about eight or nine years old Hotaru was with her father in his laboratory while he was conducting an gene-splicing experiment. Unfortunately, something with the test went horribly wrong, resulting in an explosion that destroyed the lab, killed the professor's assistants and Hotaru, and badly injured Professor Tomoe.
The explosion also had another effect in that it provided a doorway for the inhabitants of the Tau Nebula to come through to Earth. As he cradled his daughter's body, Professor Tomoe was confronted by an ominous black energy. The black energy offered to save both his life and his daughter's life if he would give it permission to inhabit both of their bodies. Driven by guilt and grief, Professor Tomoe agreed.
Hotaru's new life was filled with sorrow, loneliness and pain. She was sickly and weak, prone to seizures, fainting spells and memory lapses. She was an outcast at school because of her odd habits and because of a strange power they claimed she had; Hotaru could use it to heal, but her classmates said she used it to hurt people. Hotaru's home life wasn't much better; her father worked constantly and she was terrorized by her step-mother Kaolinite.
The only bright spot in Hotaru's life was a little girl named Chibiusa whom Hotaru had met in the park. Unlike the kids at school Chibiusa wasn't afraid of Hotaru or freaked out by her powers. Hotaru and Chibiusa became fast friends with each other. Through Chibiusa Hotaru met and befriended Tsukino Usagi and the rest of the Inner Senshi.
However, Hotaru was being watched carefully by the other three Outer Senshi, Pluto, Neptune and Uranus. They had recognized her as the reborn Sailor Saturn, a Senshi they feared because she had the power to bring about the end of the world. It was their belief that Hotaru was in league with the Death Busters, and that they were paving the way for her awakening. And they were willing to go to any lengths to prevent Saturn's awakening...even killing Hotaru.
But when the moment came it was not Sailor Saturn who awoke as the Messiah of Silence, but Mistress 9. Mistress 9 had inhabited Hotaru's body after Professor Tomoe had made his deal with Pharaoh 90, remaining dormant until Hotaru was forced to ingest Chibiusa's pure heart crystal. Mistress 9 stole the Holy Grail from Sailor Moon and transferred its power to Pharaoh 90. However Hotaru's personality had not been destroyed. She and Mistress 9 battled for possession over their shared body.
It was Sailor Saturn who emerged victorious. Hotaru's full awakening as Saturn was enough to destroy Mistress 9 completely, and she returned Chibiusa's pure heart crystal before returning to the site of the battle. She thanked Sailor Moon for protecting her before preparing to destroy Pharaoh 90 by herself using the full extent of her powers, explaining that without death there can be no rebirth. Saturn then threw herself into Pharaoh 90's core. Inside the Silence, Saturn succeeded in destroying Pharaoh 90 but as she predicted the battle killed her. Sailor Moon, powering up to Super Sailor Moon through sheer will, entered Pharaoh 90 to save her. When she emerged, she held a reincarnated baby Hotaru in her arms.
After being reborn, Hotaru was returned to the custody of her father (who had no memory of his actions under the influence of the Tau Ceti) and it was hoped that she could grow up to be a happy, healthy, normal girl. But only a few months later Hotaru was taken from her father's care by Sailor Pluto after Queen Nehelenia had re-emerged as a threat. Raised by the other three Outer Senshi, perhaps as penance for their earlier treatment of her, Hotaru rapidly aged from infancy to pre-adolescence in a manner of days before re-awakening as Sailor Saturn.
After regaining her powers as a Senshi Hotaru participated in the fight against Nehelenia. She and Sailor Chibimoon were the only two Senshi to reach Nehelenia's castle when the evil queen had abducted Mamoru. Once again Saturn demonstrated her willingness to sacrifice herself to save others, as she resolved to bring her full power to bear again and showed no hesitation when Nehelenia pointed out that she would be destroyed as well. But at the last minute Chibimoon prevented Saturn from completing the attack, not wanting to see her best friend die. This gave Nehelenia the time she needed to trap Saturn in one of her mirrors. Saturn was freed from her prison by Sailor Moon, and along with the other Sailor Senshi convinced Nehelenia to accept Sailor Moon's offer of forgiveness and healing.
Following the defeat/healing of Nehelenia, Hotaru kept herself out of the battle against the Sailor Phages, even while her foster parents aided the Sailor Senshi. She only stepped in to help after Sailor Galaxia stole the Star Seed of Princess Kakyuu, protecting Eternal Sailor Moon with her Silent Wall attack. The Senshi regrouped at the Hikawa Shrine, but the Outers slipped away and met on their own. Together they agreed on a group assault against Galaxia.
Saturn and the other three Outer Senshi infiltrated Ginga TV, the building that Galaxia was using for her headquarters, and managed to get into a control room. There they saw Galaxia holding the three Sailor Starlights hostage in her throne room. The Outers watched on the monitors as Sailor Chibi Chibi Moon transported the Inner Senshi into Galaxia's throne room, as Galaxia attempted to steal the Star Seeds of the Sailor Starlights and as the Inners took the blasts instead.
Several minutes after watching the Inner Senshi's sacrifice, the Outers were joined by Eternal Sailor Moon and the Sailor Starlights as Sailor Chibi Chibi Moon teleported them to the control room. While Uranus and Neptune decided to go fight Galaxia, Saturn tried to comfort the grieving Sailor Moon. When Sailor Star Fighter said that they couldn't win against Galaxia without the "Light of Hope", Saturn disagreed. She approached the three Starlights and asked if they liked Sailor Moon. When Fighter answered that they did, Saturn smiled and replied that they all had the same hope. She asked them to believe in their princess. Following this exchange, Saturn and Pluto followed Uranus and Neptune to Galaxia's throne room.
They reached the throne room in time for Saturn to use her Silent Wall attack to protect her foster parents from blasts from Galaxia's bracelets. Saturn proclaimed that she would destroy Galaxia even if it meant she herself would die in the process. Galaxia merely laughed and revealed that she had been the one responsible for Saturn's reawakening in the first place; she had wanted to obtain all the Star Seeds of the Solar System, but as a baby Saturn's was too immature. So Galaxia freed Nehelenia from her prison so that Hotaru would age to the point that she would reawaken as Sailor Saturn. Galaxia attacked the Outers again, this time destroying the Silence Glaive as well as Pluto's Garnet Rod.
Knowing she had the upper hand, Galaxia made the four Outer Senshi an offer: give up their Star Seeds and work for her. Saturn and Pluto refused, but after a moment's consideration Uranus and Neptune accepted Galaxia's offer. Their Star Seeds were taken and they were given bracelets similar to Galaxia's. Galaxia ordered her two new servants to take the Star Seeds of their former allies. At first Saturn and Pluto fought against Uranus and Neptune, but realized that they stood no chance as Uranus and Neptune seemed to be fighting for real. Ultimately the two Senshi stood down and allowed their Star Seeds to be stolen. Eternal Sailor Moon found her way back to the throne room as Pluto and Saturn faded away. Saturn asked Sailor Moon not to forget her trusting heart, then faded away. Saturn's Star Seed joined the other Star Seeds that Galaxia had collected, until it was freed when Princess Serenity dispelled Chaos from Galaxia's body. That Star Seed, as well as the other Seeds of the Solar System Senshi, returned to Princess Serenity and they were restored.
Once Chaos was defeated and Sailor Galaxia healed, Hotaru once again returned home with her foster parents. At their car parked on the side of the road, they watched four shooting stars (actually the Sailor Starlights and Princess Kakyuu returning home) streak by. Hotaru and Setsuna both made wishes; Setsuna asked Hotaru what she wished for, only for Hotaru to tell her that it was a secret. Hotaru then asked Setsuna what she wished for, but Setsuna slyly used Hotaru's response against her. When Hotaru asked Haruka and Michiru what they had wished for, Michiru responded that they didn't need to wish for anything as they were happiest now.