Rosco was a demon who worked for Crowley. He appeared in the Season 7 episode "Reading Is Fundamental".
While Sam, Dean, Kevin, and Meg were heading for Rufus' cabin, they stopped by a gas station for supplies. Rosco and another demon were also present, posing as truck drivers, and they spotted Meg - who Crowley had put a bounty on. Meg also saw them and recognized Rosco and his partner as demons in league with Crowley.
That night, Meg met with Rosco and his partner in the woods. Rosco asked why they shouldn't just take her to Crowley now. Meg replied she could offer them something better - Castiel, the angel that betrayed Crowley. Rosco didn't believe her at first, since Castiel was supposed to be dead, but Meg claimed he was obviously out of the loop. She then got Rosco to reveal that he and his partner were the only two demons who had followed her. Meg then stabbed Rosco's partner with a knife. Rosco tried to attack her, but Meg stabbed him too, killing him.