Hotaru first appears in Sailor Moon S as the troubled daughter of Professor Tomoe, who works at Mugen Gauken High School. The Professor originally had a kind heart, but after a horrible lab accident, Hotaru was killed, and he sold his soul to the evil entity Pharoah 90. He agreed to help them make evil Daimon eggs in exchange for his daughter's life.
Because Hotaru's mother died when she was very young, she lives in seclusion with her father in a giant mansion. We soon find out that an entity called Mistress Nine is taking over her body. (Mistress Nine is showed as a little girl sitting in a rocking chair in a room filled with toys. She displays her evilness by ripping the heads off of teddy bears *shudder*). Sailor Moon later ends up having to battle her, and is able to exorsize the demon. Hotaru is reborn as a pure soul (and she is a super cute baby-hence our name). She grows up at an alarming rate however, because Galaxia needs people to be a certain age in order to steal their starseeds.
Sailor Saturn is thought of as one of the strongest senshi because she possesses the power to destroy the world if she so chooses. She also has the coolest fuku (in my humble opinion). Her boots are awesome. Plus she's a capricorn, like me!