I get her a lot in relationship readings as well. I wouldn't always think of a mistress, she could signify different things depending on surrounding cards, the question/situation, etc.
For me, I've taken her to represent myself as seen by the other person most of the time. Also how I act, or how I appear to him.
She's attractive, dignified, social/popular... she's kind of that magnetic woman in the crowd, as I would describe her. She may also be a bit flirtatious, maybe impulsive, spontaneous, forward. Just to give you some ideas.
Hope is like a bird that senses the dawn and carefully starts to sing while it is still dark. - Anonymous
Music in the soul can be heard by the universe. - Lao Tzu