Criminal Minds + Supernatural
Selene Lillith Cain was a demon from Hell, and it's pretty obvious from her name; ironically, it wasn't her true name. She escaped Hell in 2005 and entered a dying FBI agent, taking up the role.
She never knew she could fit in again, but this team accepted her fake identity. They couldn't know what she was, so she explained to them that salt irritated her after her near death experience; so, nothing salty for her.
But what happens when Lucifer might escape the cage? What happens to her? Will she run from her "King" or stay among her team? If she does stay, will she protect them from all harm?
At any cost.
Seasons:
3 through 5 in Supernatural
3 through 6 in Criminal Minds