Hotaru Tomoe is a character in the anime/manga Sailor Moon. She is Sailor Saturn and the last to join the Sailor Senshi. Sailor Saturn is known as the Messiah of Silence, and it is feared she will destroy the world if awakened. She wields the Silence Glaive.
Hotaru Tomoe first appears in the S story arc of Sailor Moon. She is the daughter of Professor Souichi Tomoe, a mad scientist who leads the villain
organization known as The Death Busters. Hotaru's mother died when she was very young. Hotaru technically died in her early childhood in a lab accident caused by one of her father's experiments. However, Professor Tomoe made a deal with the demon Pharoh 90; in return for bringing Hotaru back to life, he would have to allow the demon Mistress 9 to possess Hotaru's body.*
Hotaru was brought back to life, but her body remained weak. However, she possessed a strange power that allowed her to heal cuts and bruises. This alienated her from people though. Hotaru was very shy and had no friends until the day she met Chibi-Usa. Chibi-Usa subsequently introduced her to Usagi and the rest of the Inner Senshi.
Sailor Saturn's awakening is feared to bring the end of the world. The Outer Senshi, Sailor Pluto, Neptune, and Uranus made it their mission to exterminate her before it could happen. However, Sailor Saturn is awakened and destroys Mistress 9--freeing Hotaru's body of evil. She also is crucial in eliminating Pharaoh 90, putting an end to the Deathbusters.
*Note: This summary draws more from the anime than the manga. There are some significant differences in her character/story between the two. For instance, Hotaru's revived body is partly made of mechanical, cybernetic parts.
Sailor Saturn was reborn as a child at the end of the S series. She reappeared in the Super S story arc (Stars in the anime). The Outer Senshi had taken it upon themselves to adopt and raise her. She is seen to have a close familial relationship with them as can be seen by her nicknames for them: Haruka-Papa, Michiru-Mama, Setsuna-Mama. She went through a rapid growth spurt, jumping years in age. She reawakened as Sailor Saturn when the threat of Nehlenia and the Dead Moon Circus emerged. She provided the Outer Senshi with new crystals which enabled them to change into their Super form.