Hotaru's Story
Hotaru started her life out with a tragic beginning. Her mother died leaving her and Professor Tomoe to fend for themselves. Hotaru grew up a happy child, she was always amazed at her father's studies in his lab. She was always lonely because she had no friends. They were all afraid of Hotaru's amazing powers to heal wounded people. One day Hotaru met Chibi-Usa by healing one of her wounds, instead of being freaked out, Chibi-Usa thought it was cool and so she befriended her. Then, a terrible lab accident killed everyone including Hotaru. Frantic, Professor Tomoe sold his soul to Pharoah Ninety to insure that his darling daughter would live. Pharoah Ninety possessed the bodies of Hotaru and her father. Hotaru swallowed Chibi-Usa's heart crystal and she became Mistress Nine. She (Mistress Nine) helped Pharoah Ninety to end the world and take over. Hotaru's love for Sailor Moon and Chibi-Usa helped Hotaru fight Mistress Nine and she freed her body from Mistress Nine's possession. By now Pharoah Ninety had already begun to end the world and Hotaru felt she must end what she had started. As Sailor Saturn, Hotaru jumped into the very center of Pharoah Ninety's galaxy and destroyed him from within. By doing that she sacrificed her life to protect Sailor Moon from having to battle. Sailor Moon tried desperately to transform into Super Sailor Moon but couldn't since Mistress Nine had broken the Holy Grail earlier. All the Sailor Senshi saw Sailor Moon's desperation to transform and their tiaras began to glow. Sailor Moon tried once again and yelled "Crisis Makeup!". She then transformed into Super Sailor Moon. Super Sailor Moon then quickly went in after Sailor Saturn to save her. A while after that she reappeared from the darkness tattered and weak, she was holding a baby. Hotaru had died and was reborn. Michiru and Haruka had taken care of baby Hotaru before they gave her back to Professor Tomoe. After that Hotaru grew up very fast to about the age she was before her rebirth. Then, one night Sailor Saturn's spirit appeared to Hotaru to awaken the sleeping Sailor Saturn inside of her. Sailor Saturn fought Nehelenia and nearly destroyed her palace. After that Hotaru doesn't appear in the show for a while until she joined the Senshi to battle against Galaxia. Galaxia takes over Sailor Uranus and Sailor Neptune and they both killed Sailor Pluto and Sailor Saturn. However, Pluto and Hotaru are both reborn and after that they live happily forever.
That is how the anime depicts Hotaru's story. The manga is a bit different, I am still reading that, I love it!