Mild-mannered librarian by day. . . slightly less mild-mannered librarian by night.
I believe the Alliance-approved term is "companion."
How about one of my favourite words, "trollop"?
I briefly considered "harlot" and "strumpet" before settling on "slattern."
strumpet is another one of my prized words. slattern is one I can't quite get into.. don't know why not- well done though.