Marjorie Willis is a librarian.
Overview
Fellow Idaho resident Jeffrey began a plan to retrieve the demon who possessed him four years ago. In order to do this, he needed the blood of the man who exorcised the demon - Dean Winchester.
Jeffrey lured Dean and his brother Sam to Idaho by killing various women, just as the demon had done in the past. He told Dean and Sam that the demon's next target was going to be Marjorie, so Sam decided to go find her and keep watch.
Sam, along with his hallucination of Lucifer, arrived at the library she worked at and sat at a table from which they could see her, however, Lucifer kept bothering Sam and tried to distract him.
When a suspicious man in a black jacket stalked Marjorie, Sam followed the pair only to discover them making out between the bookshelves. They each returned to their seats with Marjorie and the man expressing fondness towards one another.
Lucifer then pointed out that the demon was not likely responsible, and Marjorie may not be an intended victim at all. This led Sam to Jeffrey's apartment, where he discovered evidence of Jeffrey's plan.