Hotaru Tomoe was the civilian identity of Sailor Saturn, and appeared in the anime and manga series.
During her childhood, Hotaru was involved in a freak accident in her father's genetic engineering laboratory. Germatoid, a Daimon from the Tau Nebula, used an experimental space created by Professor Souichi Tomoe to cross over to Earth. His arrival caused a massive explosion that left Hotaru dead and her father injured. The sinister entity promised to restore Hotaru to life if the professor agreed to his terms. The distraught professor agreed immediately, at which point Germatoid hijacked his body, while Hotaru's body was possessed by a second entity, Mistress 9. While Germatoid's personality completely overwhelmed that of Souichi Tomoe, Mistress 9 remained largely dormant in the resurrected Hotaru, only surfacing occasionally. Hotaru tried to return to a normal life but, largely due to Mistress 9's presence, she had become very frail and prone to mysterious seizures. She also possessed unique abilities that allowed her to heal minor injuries in seconds. Meanwhile, her possessed father set about establishing the Death Busters and arranging Pharaoh 90's passage to Earth, quite unbeknownst to Hotaru.
Due to her strange abilities and inexplicable mood swings, Hotaru was ostracised by her classmates and lived a largely solitary life. However, she met Chibiusa by chance during a visit to Juuban Natural Park. After she discovered that Chibiusa was not afraid of her powers, the two became friends. The friendship was to be cut short, however, when Chibiusa's Pure Heart Crystal was stolen by Kaolinite and used to awaken Mistress 9. Hotaru's personality was pushed into the background by her evil alter ego, who succeeded in stealing the Holy Grail from Sailor Moon and transferring its power to Pharaoh 90.
Hotaru had not been defeated, however, and kept attempting to regain control of her body. Eventually, she succeeded in this by awakening as Sailor Saturn. Saturn's presence completely overwhelmed Mistress 9, destroying her permanently. After returning Chibiusa's Pure Heart Crystal, Saturn resolved to defeat Pharaoh 90 by using the full extent of her powers. Sailor Moon objected, knowing that it would mean Saturn's death. Sailor Saturn refused to relent, however, and leapt into Pharaoh 90's core. She managed to annihilate Pharaoh 90 from within, but, as predicted, the battle killed her. Fortunately, Sailor Moon entered Pharaoh 90 and retrieved a reincarnated baby Hotaru from the entity before it disappeared. Hotaru had been reborn as a baby, and was eventually returned to her father, now amnesiac but free from Germatoid's possession.
Baby Hotaru lived with her father for a while, but soon Setsuna Meiou came to take her to live with the other Outer Senshi, aware that her powers would be required for the forthcoming battle against Queen Nehellenia. When threatened by a Mirror Paredri, Hotaru's power exploded, destroying the creature and causing her to age by a few years in just a few seconds. She later continued to age at an accelerated rate, until, upon reaching early adolescence after a few weeks, she reawakened as Super Sailor Saturn, regaining the memories of her previous life. Her power also allowed Sailor Uranus, Sailor Neptune, and Sailor Pluto to achieve their Super forms, and also advanced Super Sailor Moon to her Eternal Sailor Moon form.
Her background was similar in the manga, but in the lab accident Hotaru was severely injured but not killed, and her father used his banned research to transform her into a cyborg in order to keep her alive. During this time, he also implanted her with one of the eggs that he was given by the Death Busters.
Hotaru attended her father's school, Mugen Gakuen. She was shown to be sick and an outcast, with strange mood swings (for example, when two girls picked on her, her eyes suddenly glowed and she crushed a metal pencil case in her hand). She met Chibiusa when the girl snuck onto the school grounds to find her hat, which had blown away in the wind. Both of the girls were attacked by a Daimon possessing a cat, and Usagi and Chibiusa were forced to use their powers in front of Hotaru. Hotaru promised to keep this a secret, and used a strange power to heal a cut on Chibiusa.
Chibiusa visited Hotaru several times, and learned that Hotaru still had scars from the accident in her childhood and wore long sleaves to hide them. When Hotaru collapsed from an attack from her supposed illness, Chibiusa healed her with the Silver Crystal.
The three Outer Senshi eventually told the others that Hotaru was really Sailor Saturn, a bringer of death and destroyer of worlds. In order to prevent her from awakening and destroying the world, they planned to kill Hotaru, reasoning that she would die soon anyway due to her poor health. Chibiusa went to warn Hotaru, but just then Mistress 9 awakened and stole the girl's Silver Crystal.
However, even though Mistress 9 was controlling her body, it was clear that Hotaru's spirit had survived and was watching the events that unfolded. She said goodbye to her father after Sailor Moon killed him, realizing that her true father had been long dead, and protected Chibiusa's spirit and the Silver Crystal's power from Mistress 9, keeping the entity from leaving her physical body. When Mistress 9 tried to devour the Inner Senshi's spirits for more power, Hotaru rescued them and returned them all to their bodies, then returned the Silver Crystal to Chibiusa. She faded away shortly after, though, when Mistress 9 escaped from her body.
After Sailor Saturn's apparent death as she locked away the Death Buster's homeworld, the Outer Senshi found that Hotaru had come back to life as a healthy infant. They adopted her as their own, and the "Dream" arc of the manga showed the four living together. At first Hotaru remembered nothing of her past life or powers, though she grew from infancy to childhood in only a few short months, and displayed intelligence far beyond her years. She did occasionally manifest strange abilities, such as when she had a vision of Chibiusa chasing Pegasus or created a miniature image of the solar system. As the story arc progressed, Hotaru was forced to reawaken as Sailor Saturn once more in order to combat Queen Nehellenia.
Following the Dead Moon's defeat, Hotaru went back to her life with the Outer Senshi. This time, however, she remembered her past life and had ties to the other Senshi, and was good friends with Chibiusa, who appeared to be about the same age as she was. In the "Stars" arc, Hotaru was upset when Chibiusa decided to go back to her own time to visit. When the Outer Senshi left to visit their home castles in order to protect the solar system from a mysterious invader, she was left in the care of Luna and Artemis, but sensed the danger to the Outer Senshi and vanished from Earth to protect them. A holographic recording Sailor Moon later found in Charon Castle revealed that Hotaru tried to protect Sailor Pluto from Sailor Galaxia, but both were killed. Hotaru appeared again in the Galaxy Cauldron with the other Senshi, having been rescued from Sailor Galaxia by Sailor Moon.
Hotaru's mother, Keiko, died in the same laboratory accident that injured her daughter. Hotaru lived with her father, Souichi Tomoe. In the anime, she continued to live with him after the end of the Sailor Moon S season after she was reborn, but in the manga, she was adopted by Haruka Tenou, Michiru Kaiou, and Setsuna Meiou, and she called them "Haruka-papa," "Michiru-mama," and "Setsuna-mama" respectively.
Hotaru never really aged to the point to have a romantic relationship in either series. However, Hotaru and Chibiusa were good friends in both the anime and the manga series, and she formed an adoptive family with Haruka, Michiru, and Setsuna in the Dream arc of the manga. She also lived with Haruka and Michiru in the Sailor Moon Sailor Stars season, but the "family" relationship was not present to the extent it was in the manga.
After her original appearance as Miss Dream, Hotaru soon developed a very playful personality in the musicals, very different from the other incarnations of her. This was similar to the treatment of Setsuna Meiou. She was nonetheless dedicated to her task as a Senshi.
In the musicals, Hotaru was played by Keiko Takeda, Chihiro Imai, Asami Sanpei, Mao Mita, Mario Tomioka, Ayami Kakiuchi, Ruria Nakamura, Yui Iizuka, and Eriko Funakoshi.
- The first kanji character in Hotaru's last name is also the first character in the Japanese name for the planet Saturn. Her name literally means "firefly rising from earth." Fireflies are associated with spirits of the dead in Japanese mythology.
- Hotaru's name was unchanged in the English dub; however, the pronunciation was "toe-moe" instead of "toe-moe-eh."
- In the Spanish translation of the manga, her civilian name was at first kept as "Hotaru," then translated as "Olivia," the same as in the French version, even though in the Spanish dub of the anime her name was "Andrea." In later volumes, her name was changed to match the anime.