She was a secretary working at the sheriff's office, when Sam and Dean were brought in by Victor Henricksen.
Background
Nancy, the secretary at the sheriff's office that is under siege in "Jus In Bello", is a devout Catholic. She responds to Sam's request for a towel after Dean is shot, only to be grabbed by him as he steals her crucifix. Later she helps patch Dean's wound.
Voluntarily celibate, she volunteers herself as the virgin sacrifice in Ruby's suggested plan to perform a spell and destroy the attacking demons. Dean, however, doesn't approve and comes up with an alternate plan that won't require a sacrifice.
Nancy helps during the alternate plan that they go with, and survives the siege, only to be killed by Lilith along with the rest of people in the building shortly after Sam and Dean leave.
In "Are You There, God? It's Me, Dean Winchester", the ghost of Victor Henricksen tells Dean that Lilith flayed Nancy alive.