"Small Lady is the only person you treasure above anyone else, Hotaru. But you know Small Lady is important to me, too. Even if you don't tell the others, I need to know what has happened to her."
The solitary guardian of the gate of time, Setsuna is used to spending most of her days alone in silent contemplation. Though she is no longer forced to stand guard at the gates of time 24/7 as she was in the days before Sailor Pinkmoon was able to transform, Sestuna is still loathe to leave her post without a good reason or at least a fellow senshi to keep an eye on things for her while she is gone.
After the Moon Senshi appear, Setsuna is called upon to find Seisui and bring her to Nemesis for her exile, a duty which pits her against Usako--her dear "Small Lady" who was one of the few friends to her during the time she was forced to spend her days guarding the gate of time. When Nemesis becomes corrupted and Seisui can no longer be safely held on the dark planet, Setsuna makes the case for setting Seisui free partly on her behalf.
Although Nemesis begins attacking Earth again and the senshi are called to their duty, Pluto's place seems to remain by the gate of time, silent and watchful. That is, until her priest, Mauros, appears...
For more information on Sailor Pluto from the sailor moon manga--including her attacks, forms, and items--check out her oracle article and her civilian and senshi sections on wikimoon.