Cindy McKellan was a resident nurse possessed by Lilith's personal chef in order to get babies for Lilith to eat. The demon later released control of Cindy to play on Sam's unwillingness to hurt the innocent.
Cindy was introduced by way of Ruby mentioning her to Sam as a means of finding out where Lilith was hiding. She described the demon possessing Cindy as a member of Lilith's entourage, a sort of "personal chef." As Sam was about to kill the demon, Ruby stopped him and told him he would need more blood because she could not give him all that he required herself anymore. In order to save itself, the demon let the real Cindy come forth.
The human Cindy McKellan begged Sam to let her go as he took her to the car. She talked about having a husband, and being a nurse. In the trunk she continued to scream for help. When Sam hesitated to kill her, Ruby pointed out that the demon was still inside Cindy's body and that he needed her blood to help him defeat Lilith. After listening to a message on his phone left by Dean (which, unknown to him, had been tampered with) and hearing his brother seemingly condemn for his actions, Sam decided to go through with it. He and Ruby kill the demon and her vessel, and Sam drank her blood to enhance his powers.