I'm not sure I'd rate her as high as a Mistress. More like a concubine. Actually, I did think Mistress but my husband said concubine is closer to the mark...
Well, she's a little of both, but somehow the language has never evolved a word for woman [other than the wife] with whom a married man openly cohabits, probably because the notion that a man could openly cohabit with another woman and remain married would have been unthinkable.