[@Holy Soldier]
There first was an idea with a witch, then I threw in some demon soul slavery and ending it with a quest for freedom which involved becoming a demon (for more power which conveniently gave immortality) and hunting her master to the ends of the earth. Making her land in Baylor after 400~ of traveling the world and getting cozy with the Syndicate to see what they're made of (after remembering them investigate one of her incidents during World War 2). A very long backstory but I guess something simpler could worker... :lol I'll think of something :P