Nick Munroe

A siren has been taking numerous victims before the boys arrive to investigate. Disguising itself as different women and seducing men, it gets them to kill the people most important to them. Once it realizes that it is being hunted, it disguises itself as an FBI agent named Nick Munroe and makes friends with Dean, pretending to share his tastes in music and cars. Eventually the siren infects Dean, convincing him that it should be Dean's brother, rather than Sam. By the time Sam returns to the hotel room, Dean is fully under Nick's power and is prepared to kill Sam for the siren. The siren then infects Sam and pits the brothers against each other.