She appears at the end of Sailor Moon R. Her powers are over the control of time.
Setsuna likes to shroud herself in mystery. As the guardian of the passage of time, Setsuna is very isolated, and only likes to reveal herself in the most desperate situations, although as the series goes on we see more and more of her.
Unlike the other scouts, Setsuna is much older (although her appearance is still as a young adult due to her existance in the passage of time).
In Sailor Moon R Pluto is the one who helped Rini to go back in time to try and help her mother (Sailor Moon). As Sailor Pluto, she wields the Time Staff (it looks like a very long key). In her staff is the third talisman, the Garnet Orb.