Setsuna is the
oldest of the Sailor Senshi and the first Outer Senshi
that appeared in the series, more exactly in SMR as the
guardian of the time gate. Though, she goes out of the
dimension it is in, a lot of time.
There isn't too much to say about Setsuna. She's great with children: as you can see Chibi-Usa loves her very much and Hotaru also. Love life? None. Though there are rumours of her being in love secretly with King Endymion, Tuxedo Masked (etc) but they are just rumours. (At least in the Anime, in manga she could be...) In SailorStars she lives with Michiru and Haruka raising Hotaru. She calls her Setsuna-mama. She's the most sophisticated of the Sailor Senshi and has like 1000 years old because she has lived in the time gate for who-knows-how-much years.
Her name means Moment/Instant Dark Ruler. Pluto is called in Japanese Dead King Star but though this fact, Pluto is more related to time and change. But, this explains her Dead Scream attack. She has one of the talismans, the Garnet Orb, which she carries in her key-shaped staff. In SailorStars she turns in Super Sailor Pluto.