When she was a child, Hotaru was involved in a freak accident in her father, Professor Souichi Tomoe's lab. The arrival of a Daimon named Germatoid caused a massive explosion in the lab due to Professor Tomoe using an experimental space that he created. Hotaru was severely injured but wasn't killed. To keep her alive, her father turned her into a cyborg using his banned reaserch. An egg(Mistress 9) from one of the Death buster was also implanted in her during her tranformation. Due to this, Hotaru was always seen wearing long sleeves and dark tights to cover herself. She also always appeared to be sick and had frequent, strange mood swings. Hotaru was always picked on when she later attended her father's school, Mugen Gakuen. Chibiusa was the one to became her first real friend along with Usagi. Eventually everyone learned from the Outer Senshi that Hotaru was really Sailor Saturn, the Senshi of death and rebirth. Afraid of what she will become, the Outers planned on killing her. Though it was a strenuous, tough battle, Hotaru, still fully clear of what was going on, rescued the Senshi when a fully awoken Mistress 9 tried to devour their spirits and returned the stolen Silver Crystal to Chibiusa. She disappeared shortly after when Mistress 9 escaped from her body. She was later found by the Outer Senshi to have comeback as reborn infant who they adopted. It wasn't until way later on that she and her intelligence would grow at an alarming rate and finally reawaken as Sailor Saturn.
In the anime, Hotaru's background is similar to the manga. But instead of being severely injured in the lab accident, she is killed. The Daimon, Germatoid promised Hotaru's father, Professor Souichi Tomoe that he can bring her back to life if he can agree to his terms. Desperate, her father agreed immediately. Soon after, the entity took possession over the professor's body while the other entity, Mistress 9 possessed Hotaru's. While Germatoid's personality completely took over that of Souichi Tomoe, Mistress 9 remained largely dormant in the resurrected Hotaru, only showing herself every once in a while. When trying to return to a normal life, Hotaru became very frail and was prone to mysterious seizures, largely due to the presence of Mistress 9. Also possessing unique abilities, Hotaru was able to heal minor injuries within seconds. She became outcasted by her classmates due to her strange abilities and inexplicable mood swings, and lived a largely solitary life.
Hotaru, by chance, however met Chibiusa during a visit to the park. They soon became friends after Hotaru rescued Chibiusa's hat, which blew away from the wind and realized that she wasn't afraid of her abilities. Unfortunately, Kaolinite found out that Chibiusa possessed a Pure Heart Crystal and stole it from her to awaken Mistress 9, causing Hotaru's and her friendship to be cut short. Hotaru's personality was pushed into the background by Mistress 9, who the succeeded in stealing the Holy Grail from . Mistress 9 then transferred its power to Pharaoh 90.
Hotaru however, still not defeated, kept trying to regain control of her body. She succeeded in this eventually by awakening as . Completely overwhelmed by Saturn's presence, Mistress 9 was destroyed completely. Sailor Saturn, after returning Chibiusa's Pure Heart, resolved to defeat Pharaoh 90 by using the full extent of her powers. Sailor Moon objected, knowing that it would mean Saturn's death and the loss of Chibiusa's friend. Refusing to give in, Sailor Saturn leapt into Pharaoh 90's core. Managing to annihilate Pharaoh 90 from within, Saturn, as predicted was killed in the battle. Fortunately, Sailor Moon entered Pharaoh 90 and retrieved a reincarnated baby Hotaru from the entity before it disappeared. Hotaru, having been reborn as a baby, was eventually returned to her father, now amnesiac but free from Germatoid's possession.
Powers and AbilitiesEdit
In both the manga and anime, Hotaru showed special abilities even when in civilian form, such as the ability to heal minor injuries. In the anime, however, she was also able to affect several Daimons in various ways: she created a protective barrier around herself against U-Tomodachi, used what seemed to be an electric shock against U-Baulla, repelled U-Ndokai, and froze U-Chouten in place. Her abilities in the anime could have been related to the as-yet-unawakened power of Sailor Saturn within her, or they could have been because of the presence of Mistress 9.