Jocasta Nu

Jocasta Nu, a.k.a. The Grey Lady, was a free-floating, full-torso, vaporous apparition that tended to the Jedi Archives in the Jedi Temple on Coruscant. In days of yore, like, five thousand years ago, she was a detail-oriented, sedentary Jedi who spent most of her time tidying up the card catalog and reading Agatha Christie novels. She lived to the ripe old age of four hundred, died peacefully, and never noticed. Her Force Ghost then went on as head librarian for centuries.

At one point, Obi-Wan Kenobi encountered her while hunting through the archives on Kamino. Jocasta manifested behind him just long enough to assure him, "If an item does not appear in our records, it does not exist." She then floated away and coated several precious Jedi holocrons in ectoplasmic goo.

Eventually, around the time of the founding of the New Republic, Jocasta's restless ghost went insane and began scaring the crap out of everyone, prompting the Jedi to hire the Ghostbusters to dispose of her.