Meg's vessel

Meg's vessel was an unnamed girl who acted as the demon Meg's vessel from season 5 to her death in season 8.

Background
Not much is known about this girl except that she was a girl from Cheboygan, Michigan who wanted to be an actress. She may have had a hard life as Meg stated that being tortured by the demon possessing Christian Campbell wasn't even the worst that the girl had gone through. At some point following her possession of Sam Winchester, Meg possessed this girl's body to use as her vessel.

Sympathy for the Devil
Meg reappears after a year and a half wearing this girl's body. After Dean Winchester and Bobby Singer kill her demon minions, Meg flees the girl's body to avoid getting stabbed by Dean with the demon-killing knife herself, leaving the girl unconscious or dead on the floor.

Caged Heat
At some point following her fleeing the girl, Meg repossessed her and reappears a year and a half later to force Sam and Dean to help her hunt Crowley. During the hunt, she is tortured by the demon possessing Christian Campbell and tells him a bit about the girl's past and taunts him with the fact that the torture isn't worse than anything the girl had gone through in her life. When the group is faced by hellhounds, Meg tries to flee the girl again but is blocked by Crowley's spells. While Meg flees once more after Crowley's "death", she takes the girl with her this time.

Goodbye Stranger
Meg is rescued by Castiel from torture by demons loyal to Crowley and due to the torture, the girl's body has taken a beating. In the end, protecting the Winchesters and Castiel from Crowley, Meg is killed by him with an angel blade. If the girl was still alive inside her body, she was killed too. The girl's lifeless body is last seen lying with the bodies of all the other demons slain by Sam and Meg to buy Dean and Castiel time to escape with the angel tablet.