Hotaru is one of those characters that you feel sorry for. After a lab accident where she was killed, her father agreed to sell his soul to restore Hotaru's life. But then the person who did this started to take over Hotaru too..... On top of this, Hotaru didn't have any friends until she met Chibi-Usa (Rini). This was because she once used her healing powers in public to help someone, but then everybody thought she was weird and stayed away from her. However, when Hotaru told her this, Chibi-Usa thought that was an amazing power and was impressed with Hotaru.

Hotaru's father is Professor Tomoe Souichi, a brilliant scientist and director of the Tomoe Research Institute, and also the owner of Mugen district. Her mother, Keiko was killed in a fire accident when she was only eight years old. Hotaru has a dual personality in Sailormoon S because she was possessed by a powerful female evil spirit called Mistress 9. Part of her is Hotaru herself but the other part is Mistress 9, so this is actually two people in one body. This is the reason of Hotaru's constant sickness and mysterious nature. No one dared to be her friends because they feared her, especially of her mysterious powers. Fortunately, she found friendship in Tsukino Usagi, and developed a very close relationship with Chibiusa, who later became the inspiration in Hotaru's battle against Mistress 9 within her own soul. Sailor Saturn did not appear until the ending part of the Sailormoon S series. The other Sailor Senshis did not know that Hotaru was possessed by Mistress 9 since Professor Tomoe was forced to join the Death Busters to save his daughter from dying after a terrible explosion in his laboratory.

Haruka, Michiru and Setsuna (Outer Senshis) had an animosity towards Hotaru because they could not understand the origin of the mysterious aura around her body. At the end of Sailormoon S, the Sailor Senshi discovered that inside Hotaru lies the Sailor Saturn spirit, the senshi of destruction, with powers that could destroy an entire world. The Death Busters thought of this as an excellent opportunity to utilise the power for their purpose through Mistress 9. Sailor Uranus and Sailor Neptune warned the Inner Senshi that they would destroy the evil spirit before Sailor Saturn awakens, but the Inner Senshi refused as this might be at the expense of Hotaru's life. At first Uranus and Neptune were right as Mistress 9 finally awakened and managed to prevail over Hotaru's spirit, grabbed Chibiusa's heart crystal, tricked Sailor Moon to obtain her Holy Grail to harness her energy, and then was on the verge of calling Master Pharoah 90, the ruler of the Death Busters into the world. However, Hotaru climbed back from near-oblivion, fought back and finally succeed in defeating Mistress 9. Hotaru turned to Sailor Saturn, gave back the heart crystal to Chibiusa and battled against Master Pharoah 90. However, using the powers of destruction would mean Hotaru's death. Sailor Moon managed to transform after a few vain attempts into Super Sailor Moon without the Holy Grail, thanks to the combined powers given by the rest of the Sailor Senshi, and saved Hotaru's life. Somehow the girl was reborned as a little baby. Baby Hotaru was later left to Haruka and Michiru, and then to Professor Tomoe, who lost his memory of his sinister past and finally regained his true self, for care.....