Meg Masters (Supernatural)

Background
She was a human being. Her old life was unknown. In hell, she trained under Alastair who taught her ways of how to torture her prey.

Season 6
Meg along with Castiel, Sam, and Dean hunt for Crowley. Before Meg takes on a Hell Hound, she kisses Castiel. Castiel grabs Meg and takes the lead in front of Sam and Dean who are surprised. When Meg asks Castiel where he learn how to kiss like that, Castiel replies he got it from the pizza man.

Personality and Traits
She was a cruel demon who stopped at nothing in achieving her goals. She killed many of the Winchester's allies in an attempt to carry out her missions. After the apocalypse arc, she had a change of heart once she became friends with Castiel. There are strong implications of a romantic interest in Castiel as noted during the pizza man incident. She sacrifice herself to buy time for Castiel and the Winchesters after Crowley had killed her.