Tomoe Hotaru is the Pretty Soldier Sailor Saturn. Hotaru is quiet, submissive, and doesn't have many friends besides the other senshi and Chibiusa. Her father, Professor Souchi Tomoe is a scientist. During the presentation of one of his machines, the machine exploded killing everyone including Hotaru and her mother Keiko, except for Tomoe himself. An entity (Mistress 9) came to Tomoe and told him if he let the entity possess him, he would bring Hotaru back to life. He readily agreed, for Hotaru's sake, but turned into a mad scientist, performing all sorts of experiments, some on Hotaru herself. Hotaru is constantly weak, but she has a special healing power which is why some of the children her age are afraid of her.
In the S series, Hotaru is quiet and mostly unhappy, but in SailorStars she is out-going, friendly and energetic. In the manga, Hotaru as a different personality as well. The manga even suggests that Hotaru is half-cyborg, due to her father's experiments.
Hotaru is also known as the Dark Messiah. When she is given Chibiusa’s heart crystal, she transforms into the entity Mistress 9. Mistress 9 isn’t Hotaru, though, and Saturn awakens from inside of her and defeats Mistress 9. As Saturn, Hotaru is one of the strongest senshi. She has the special power of rebirth, and when she sacrificed herself at the end of S to defeat Master Pharoah 90, she was reborn again into a little baby.(cute ^.^) It is arguable whether or not she or Moon is the most powerful senshi. Saturn has the power of destruction, whereas Sailor Moon's is based on purity, or even creation. So, to keep things simple, Saturn is the strongest of the Outer senshi.

Attacks

Saturn Planet Power, Make up: Transformation phrase, although she doesn't have a transformation sequence
Silence Wall: her attack that is an energy wall
Silence Galive Suprise: attack using the Silence Glaive
Death Reborn Revolution: her attack that can destroy the world
*Note*
Saturn is never actually seen doing any of her attacks in the S anime

Saturn Planet Power, Make up
Silence Wall
Silence Galive Suprise
Death Reborn Revolution