Casey

Casey was a demon possessing a bartender in Ohio. Unlike her other demon brethren, she did not do any direct harm to humans. She also was willing to follow Sam if he chose to lead the demon army. The demon possessing Father Gil is her husband.

Background
Casey was a bartender in a small town in Ohio that was plagued by vice. When Dean saved a man in her bar, she flirted with him a little before he left. She later took Dean's friend Richie to the house her parents left her, where she killed him, revealing that she was in fact possessed by a demon. The next day she brought Dean there instead, intending to kill him as well, but he was a step ahead of her: having tracked the GPS in Richie's cell phone, he found the house, buried Richie, and made a Devil's Trap on the floor, concealing it beneath a carpet. He tried to banish the demon but before he could complete the ritual she caused a small earthquake, scattering the pages of the exorcism book and trapping Dean in the basement with her, waiting on either Sam or Casey's backup to arrive first. The two enemies decided to be civil with each other since they were at a stalemate (Dean could not banish her without his book and she could not hurt him while in the Devil's Trap), and struck up a conversation. Eventually, the two reached a sort of friendly understanding, or at least an amicable tolerance. Casey respected Dean's deal and Dean regarded her with a tolerance he does not afford demons. When Dean calls her evil, Casey responded that she did not kill anyone, simply saying that one can "nudge humans in the right direction and they’ll walk right into hell with big fat smiles on their faces". Casey identified herself as a true believer in Lucifer, comparing him to the Christian's God, and told Dean the Yellow Eyed Demon's name, Azazel, and that Sam was supposed to lead the demon army.

Father Gil ended up arriving first, having left Sam upstairs. She kissed him passionately after he freed her from the Devil's Trap and then told Dean that they had been lovers for centuries. When he wanted to kill Dean, she quickly begged Father Gil not to before Sam charged in with the Colt, killing him. Dean similarly hoped to spare her, but Sam fired a fatal shot at her before he could stop him.

Appearances
"Sin City"