Sarah was Nick's wife. She was murdered during a home invasion.
Not much is known about her, except that somewhere in time she married Nick and had a child. At an unspecified time after, the family excluding Nick were all murdered during a break-in.
In 2009, Nick (still in grief for both his wife's and baby's deaths) was haunted by hallucinations caused by Lucifer, who had just been released from his cage. In a dream, Lucifer appeared as Sarah and tried to convince Nick to say Yes, saying Nick didn't deserve what happened to him and it was God's fault in the first place. Nick, skeptical that it was really Lucifer, asked if he could bring back his deceased family, but Lucifer said he couldn't, but could give him justice.