Hotaru is first introduced when she becomes friends with Chibi-usa. She did not have friends until she met Chibi-usa and Usagi. In the anime she was not popular with her classmates because of her powers and her strange seizures as well as personality changes for being the host of Mistress 9, the evil entity that lived in her after the age of 5. Naoko Takeuchi, the series creator, describes her as delicate, quiet, precocious and expressionless.
In the manga, there was a lab accident which killed Hotaru's mother, Keiko, and critically injured Hotaru. Souichi Tomoe, Hotaru's father, rescued her by making her body cybernetic and making a deal with Master Pharaoh 90. He used Pharaoh 90's power in the hope of reviving her. In exchange, he also lost his humanity and became malevolent. After Sailor Moon prevents Sailor Saturn from destroying the world at the end of Infinity she is reborn as a baby without the cybernetic parts. She also ceases to have seizures after this point.
In the anime, she does not have a cybernetic body and is an outcast because of her powers and seizures. Her mother is deceased. Her father does not die like in the manga, but lives after the end of Super. He takes care of Hotaru until Sailor Pluto asks for her in the first part of Sailor Stars. It is unknown if she returns to her father after the battle.
Hotaru's age fluctuates during the series; she is first introduced as a young teenager, is reborn as an infant, quickly develops into a young child, then grows up to be an older teenager.
As Sailor Saturn, Hotaru can destroy a planet. Her powers awaken once the three Talisman of Sailor Uranus, Neptune, and Pluto gather and resonate. Saturn reveals in the Infinity arc of the manga that this power is necessary, because rebirth cannot happen without death. It was Saturn who destroyed the remnants of the war-torn Silver Millennium kingdom, allowing a new world to be born. At first, knowledge of her powers caused the other Outer Senshi to attempt to kill her unawakened form. However, Saturn's awakening ultimately saved the world by forcing Master Pharaoh 90 to escape to his former universe before being destroyed with Earth. Sailor Pluto then sealed the gateway to their dimension so that he could never return and Super Sailor Moon used her powers as Neo-Queen Serenity to resurrect all that had been destroyed.
Hotaru wants to be a nurse one day. Her hobbies include reading and collecting lamps. She enjoys her World History class, but dislikes Physical Education and has difficulty with marathons. She also hates milk. Other favorite things include soba (buckwheat) noodles, panthers, and the color purple. Her blood type is AB, and her height fluctuates throughout the series depending on her present age.
In the manga, after Professor Tomoe's death, Hotaru becomes the adopted daughter of Haruka Tenoh, Michiru Kaioh and Setsuna Meioh. She refers to them as Haruka-papa, Michiru-mama and Setsuna-mama. They live together in an Empire Victorian House.
In the musicals, Professor Tomoe is only present in a few stages, though Sailor Saturn appears in every musical from the third on. In musicals where Tomoe is absent, Hotaru is presented as being in the care of Haruka and Michiru.
While several Sailor Senshi have unusual powers as civilians, Hotaru's are numerous and varied. Some of these are portrayed only in the third story arc, when she is possessed by Mistress 9; these include forcefields and energy beams that arise when she is in danger, often involuntary. As shown in that arc, she also has the ability to heal minor flesh wounds. In both the manga and anime she is reborn as an infant immediately upon fulfilling her task as Sailor Saturn, and later, when she is needed in that role again, grows to her former age in a matter of days. Eventually she is visited by an image of herself in Senshi form, who bestows power on her again.
In the manga, Sailors Uranus, Neptune and Pluto gain their second transformation when Hotaru gives them their Sailor Crystals after she has regained her older body. In the anime, Hotaru is still a baby when her power floods Sailors Uranus, Neptune and Pluto, upgrading them and herself to their Super forms. In both versions, Hotaru has visions of coming events, especially of danger, and can project these visions to other people. When she is a young girl, she creates a small projection of the galaxy which hovers in the air in her room, simulating astronomical history at high speed. However, it is only in the manga that Haruka stays with her while she reproduces these events to help keep her power in check.
Her transformation into Sailor Saturn is rare. The transformation sequence into her Senshi form is never shown in the main series, although one of the video games included a shortened anime-style transformation sequence, which she initiated by raising her hand in the air (like the Guardian Senshi) and shouting "Saturn Planet Power, Make up!" (like the Outer Senshi). In the manga she eventually gains her Saturn Crystal and does transform in front of the other Senshi, evoking Saturn Crystal Power! Make Up!. In the anime, although she upgrades to Super Sailor Saturn, the Crystal is never mentioned and her transformation is again not shown on screen.
Sailor Saturn's powers are mostly based on Roman mythology, in which Saturn is the god of the harvest. As such, her primary role is that of the destroyer, and her coming is greatly feared by the Outer Senshi. Her first act upon arrival is to assault Master Pharaoh 90 with the attack "Death Reborn Revolution." The kanji used with this maneuver translate to "death world revolution" , with the intended pronunciation given in furigana.
The item most associated with Sailor Saturn is her iconic Silence Glaive, which, apart from its capacity as a bladed weapon, figures in many of her named attacks. It is referred to once as the "scythe of the Goddess of Death," and the act of lowering its point to the ground is all that is required for her to "bring ruin to the world." No incantation is spoken, and this action is separated from "Death Reborn Revolution" by the dialogue given in between the two maneuvers. In the anime, it is implied that Sailor Saturn will die if she uses this ability, and Super Sailor Moon risks her life to prevent Saturn from doing so, defeating Master Pharaoh 90 herself. In the manga, Sailor Saturn deliberately allows herself to be sucked into Master Pharaoh 90's dimension while he is escaping.
When Sailor Saturn returns near the end of the fourth story arc (which overflows into the beginning of the fifth anime series), several more attacks are introduced. She defends herself and her allies with Silence/Silent Wall , a dome of dark energy that deflects attacks, and attacks with Silence Glaive Surprise . In the anime this attack is used against Nehellenia; seeing no other way to defeat her, Saturn intends to bring down her Glaive and destroy everything, but before she manages to strike the ground Sailor Chibi Moon throws herself at Saturn to stop her from completing the action and sacrificing her life, nevertheless, the Glaive releases massive power that destroys most of Nehellenia's palace.
In the manga series, the Death Busters use a power source called the Tioron Crystal, which was once worn by Hotaru's mother, Keiko Tomoe. After her death, Professor Tomoe gives it to Hotaru in order to ease the pain she must frequently endure. The Saturn Crystal is her Sailor Crystal and the source of all her power, which becomes especially important in the fifth story arc.