SailorPluto is the senshi of space-time, and has lived longer than any of the other senshi. (Apparently she lives between time... but we won't go there, because it confuses me.) She made her first appearance in the manga volume 5, the solemn, quiet guardian of the Space-time Door. She carries the Garnet Orb, one of three Talismans.
Of all the senshi, she is the only one that has never been reincarnated; she alone has lived through the ages. She is tall, serene, and very reserved. Her skin is dark, her long hair dark pine-green (sometimes black), and her eyes are jewel-tone garnet, a deep red. She always has her hair up in a partial bun, the rest of it flowing down below her waist.
She and Chibiusa are very close friends. Chibiusa calls Pluto "Puu" and looks to her for guidance and support. Pluto first appeared as the senshi who helped Chibiusa move through time; she calls Chibiusa "Small Lady."
Queen Serenity once forbade the powerful young senshi Pluto three things, although they were within her power. First, she may not cross time. Second, she can never leave the time-space door that she must protect. And third, although she and her Garnet Rod can move within time, she must not stop it. If she ever does, she will lose her life. In the manga volume 6, when Prince Diamaund of the Black Moon gained possession of both past and present Crystals, Pluto halted time to stop him from destroying everything. (Yes, like the fool he was, he was going to destroy the world.) Go figure. So she gave up her life to stop Diamaund, buying time for Usagi and Mamoru to recover.
This was actually a good thing, since her death shocked Chibiusa out of the form of the Black Lady. Later, the power of the Millenium Silver Crystal brought Pluto back to life, and she traveled into the past to work with the other outer senshi. In the normal world, Pluto is Meioh Setsuna, a nurse or a research assistant or something similar... although she is also apparently a theoretical physics major...
Sailor Pluto is also, of course, the Princess Pluto, holder of the star seed of Pluto. The translation of her everyday name, "Meioh Setsuna," hints at time by translating to "Moment Dark Ruler"... but then one can never be very precise when translating between languages.
Pluto is of the blood of Chronos, the Time God. It is rumored that she feels an unrequited love for Endymion, but that she had to put aside her feelings in deference to Serenity. This is hinted at in the manga, where Chibiusa notes that Pluto smiles rarely, and usually at Endymion... Also, there's those scenes where she's very pleased when he complements her, and she kneels to him ... However, there's nothing really substantial to give this rumor any weight. And no one would ever think of asking grave, dignified Pluto what her feelings are on this matter...
Many thanks to Ian Andreas Miller, who gave me the correct translation of Pluto's senshi title and the name of the portal she guards.
For a closer look at Pluto...