Sailor Pluto is probably the most mysterious of the Outers. This is in part because her mission as the guardian of the gates of time do not allow her much time to do other things. A lot of responsibility has been placed on the shoulders of one woman, yet she handles her situation in a sort of stoic grace.
Sailor Pluto first appeared in Sailor Moon R, as Chibusa's guardian, and the two communicated with each other by way of the Luna Ball. Through the Luna Ball, Pluto counseled the young girl that she should stay in the past, where it was relatively safe, and that it was imperative that she find the ginzuishou.
Once Chibusa befriends the Sailors, they discover a plot by the Dark Moon family to take over Crystal Tokyo. (I know I am glossing over entire episodes here...) Sailor Pluto instructs the Sailors on how to get to the future, and further assists them with information when Chibusa is being terrorized by the Grim Man.
Although they are not brought out in the dub, Sailor Pluto has some powerful abilities to aid her in making sure the flow of the timestream is as it should be. Her powers aren't well defined in the anime, although she does use her most powerful weapon, her ability to stop time with the utterance of two words, "Time Stop!" This is a power that Pluto is generally forbidden to use, and it costs her her life...at least until the next series when she is resurrected or simply reappears.
"Sailor Moon, The Role-Playing Game" describes Pluto's powers this way:
"She has been charged to guard the Gate of Time with her key-shaped Garnet Rod. Sailor Pluto has the ability to stop time (although using this power is forbidden, to correct time distortions, and to see into the future, but she usually does not divulge what lies ahead."
Sailor Pluto also allows her allies to travel through time.
Sailor Pluto Links
Sailor Pluto Webring