kept girl
A female that is "kept" by a wealthy person in case their current relationship / marriage falls through. They are often paid a yearly salary to stay on retainer.
The kept in "kept girl" implies two things: One being that they are well kept for financially, the other being that they are kept by what could be considered their owner, ie. they are "keeping" her.
"She's a kept girl."
"She's a kept girl."