Priscilla Penn Cropper was Mortimer Cropper's great-grandmother. She was a contemporary of Christabel LaMotte and Randolph Henry Ash, and, as Possession's narrator eventually reveals, she corresponded with both of them.
We readers never get to see the letters that Priscilla wrote to Randolph and Christabel; instead, we only set eyes on Randolph's and Christabel's letters to her. As a result, we have to form a picture of Mortimer Cropper's great-grandmother by reading in between the lines of Randolph's and Christabel's letters—and by listening closely to Mortimer Cropper's own descriptions of her as he imagines the autobiography that he might like to write someday (6.27).
Although the novel's descriptions of Priscilla help to add texture and detail to its depiction of nineteenth-century spiritualism, her most important role in Possession is to give Mortimer Cropper reason to feel that he has historical connections to both R. H. Ash and Christabel LaMotte. The connection to Christabel LaMotte is particularly important, as it helps to put Cropper on Maud Bailey and Roland Mitchell's trail.