Hotaru is a sweet, gentle, and rather grave girl. She lives in a rich area of Tokyo, and attended the exclusive Muguen Gakuen Academy. There, Hotaru didn't have many friends because they thought her odd and her unusual power to heal strange. She also is frail, and tires very easily. She lived alone with her father, Professor Tomoe Souichi who used to be good until a lab accident in which many things happened (heh, what a spoiler). Hotaru, however, remained a kind girl, and in Sailormoon S, Chibiusa befriends her (the focus of Sailormoon S). They become best friends.
In S, Mistress 9 is found to be part of Hotaru. As Mistress 9, Hotaru has very very long hair^^, wears a deep purple dress, and acts very unlike her real self. But, Hotaru gets the better of M-9, of course, and she becomes a good senshi.
As Sailorsaturn, Hotaru is very powerful (despite her physical weakness.) She can be considered second to only Sailormoon (especially if you are biased towards the outer senshi^^). However, her attacks aren't really seen very much as she gets limited airtime. Even as the focus in SailormoonS, the manga incorporated her more.
I don't think she even got a transformation sequence. :(

Sailorpluto is the oldest senshi. She holds command over time, and her attacks / powers, including a forbidden power allowing her to stop time, make her a dangerous senshi.
Like Sailorneptune and Sailoruranus, she possesses a talisman, the Garnet Orb. It doesn't really give her any sort of attack, but it ornaments her time staff with which she infrequently whacks people. ^^
Sailorpluto first appeared in SailormoonR as the voice inside Chibiusa's LunaP. (She sent Chibiusa back in time to find the lost ginzuishou.) Her duties were / are to guard the gateway of times so that the future is not disrupted. It is a lonely assignment, and Sailorpluto must have stood alone at the doorway for centuries until Chibiusa in the future (Small Lady) visited, and they became friends. (Chibiusa makes friends w/ every lonely person, just like Usagi). Chibiusa calls her "Puu".
As Setsuna, she is 18 or so for the most part, and she likes to shop 8) She dreams of designing clothing, and is always nicely dressed. (One wonders what she thinks of the sailor fuku :o)