Prior to the story, when Hotaru was eight years old, she lived with her parents in Tokyo's Sankakusu District when a freak storm occurred causing a fire that killed her mother Keiko and left her on the verge of death herself. Later admitting that she was meant to die at the time, Hotaru's life was unnaturally prolonged by both the actions of her father Souichi Tomoe turning her into a cyborg and being made a host to the Daimon Mistress 9 whose presence caused Hotaru's body to suffer seizures with only the Tairon Crystal appearing to reduce the pain. Wearing clothes to conceal her altered body, unaware of Mistress 9's existence prior, Hotaru was friendless before meeting her best friend, Chibiusa.