The husband returned, but

I've brought something infuriating again

This time it's Briel, I didn't write down what to call her...

Mistress/Homewrecker - Too much like a modern melodrama
Lover - Seems too lighthearted
Sweetheart - The word seems too affectionate for an affair
Concubine - Feels more like Korea's Joseon Dynasty than a Western fantasy romance

I'm asking you, users, what should I call these kinds of people?