Hotaru is the only child of Professor Tomoe. She is a very shy and isolated person, who doesn't make friends easily. In her normal form, that is as Hotaru not Sailor saturn, she has the ability to cure wounds. This power has been her nemesis in the social scene, her classmates claiming that this odd ability of hers is unnatural and gross. This has led to her spending most of her time in her father's mansion.
She also has a weak body. She can exhaust herself much quicker than any normal person, partly as a tradeoff to her ability to heal.
As Sailor Saturn, she is not her normal, well spirited self. As the vessel for the entity known as Mistress Nine, her only goal is to destroy all life on Earth. She also wields the fearsome polearm known as the Silence Glaive. She can use the weapon to slice through anything that gets in her way.
However, at the end of Sailor Moon S, she is exorcised by Sailor Moon and the evil influence of Mistress Nine is thus removed.
Throughout Sailor Moon Super S, Hotaru is but a baby, reduced to this by her contribution by assisting Sailor Moon in saving the world in the climax of Sailor Moon S.
It is only in Sailor Stars when Hotaru rediscovers her powers and grows back to her normal self. But this time, she fights for the cause of good along with the rest of the Scouts.