When Hotaru first appears in the manga, we find out that she has a healing ability that her classmates find strange, and thus, has become somewhat of a loner, being branded as 'weird.' However, as she healed Chibi Usa after a battle, they soon became friends when Chibi Usa sought an acquaintance. In both manga and anime, Chibi Usa is apparently the closest friend Hotaru has had.
On another note, Hotaru's past and role as a Sailor Senshi is quite complicated. According to the manga, when she was 8 years old, there was an accident -- presumably a fire -- in which her mother died and she received heavy injuries. To save her, her father Professor Tomoe, a scientist, fused her body with robotics of a sort. However, the Professor also had ulterior motives, as in turning Hotaru into an android, he was also able to further his research. In the midst of his new research interest, the villians in Sailormoon S made their invasion. His project thus destroyed, Tomoe began researching on producing the ultimate 'super being' which was to be several times more powerful than a regular human being.
Finally, in manga 9, we find that due to Professor Tomoe, Hotaru had been hosting Mistress 9 within her body. After Sailor Moon used the Holy Grail to turn into Super Sailor Moon, Mistress 9 awakened and Hotaru's physical being ceased to exist, although her spirit still lived within the body of Mistress 9. In the final battle between Pharaoh 90 (Mistress 9's master, so to speak) and the Senshi, Sailor Moon thought to unleash the power of the ginzuishou within Pharaoh 90's body. This was pretty much suicide until Sailor Saturn awakened, and being the Senshi of Death, sealed up the villians in their own time and space. Sailor Moon then left Pharaoh 90's body, became Princess Serenity, and pretty much set everything right.
In Hotaru's case, in both manga and anime, she 'died' because in using her powers to defeat the villians, she also destroyed herself in the process. But, with the power of the ginzuishou, she was reincarnated almost instantly, and in the form of a baby, was put into the care of the Outer Senshi.