"The Moon Senshi are young senshi, and they have a dark history to contend with. Their misdeeds are not half of yours or mine, Haruko, so it would be wise not to judge them too harshly."
Hotaru first befriended Chibi-Usa when she was a young girl training to be a sailor senshi in the past. At that time, Hotaru was the unwitting pawn of the evil entity known as Chaos, who used her as its vessel. Chibi-Usa helped save Hotaru with the help of Sailor Moon--both from the evil entity that was possessing her, and from Sailors Uranus and Neptune, who believed that if Hotaru were to awaken as Sailor Saturn, she would bring about the end of the world.
Eventually, Hotaru was healed of her taint and became a powerful and trusted member of the sailor senshi. Now, in the future, Hotaru is reunited with Usako as friend once more now that her friend is of age and the two can have a relationship closer to that of equals.
But with the appearance of the Moon Senshi, Usako and Hotaru's relationship comes under some strain and Hotaru finds herself torn between the demands of Endymion--who wants her to keep Usako out of trouble--and Usako herself, who wants to be treated more like an adult now that she has powers and a team of her own. At the same time as she's forced to play a delicate balancing act between duty and friendship, Hotaru finds herself sympathizing with Seisui and her dark past--a dark past that contains echoes of Hotaru's own...
For more information on Sailor Saturn from the sailor moon manga--including her attacks, forms, and items--check out her oracle article and her civilian and senshi sections on wikimoon.