They are often written as a foil to the heroine: vain, overblown, and vengeful. They often have some skeevy characteristic such as malice, possessiveness, or extreme avarice that only manifests itself (or seems unattractive and potentially problematic) to the hero after he meets and/or falls for the heroine. In nearly half the novels in which they make an appearance, they don’t take dismissal by the hero with a thank you, but rather use it as kindling in the formation of a plot to harm the heroine.
Beware the ides of Mistress.
Wife in Water Colours (noun)
A mistress, or concubine; water colours being, like their engagements, easily effaced, or dissolved.
I know many Regency gentlemen kept mistresses, and I have no problem reading of their accounts in contemporaneous resources and historical texts. I don’t, however, want to read about them in flagrante delicto with the hero in my historical romance. The hero may visit her off-page, give her her congé, or even offer assistance toward a more respectable direction; I don’t want to read about them engaging in energetic discourse of a horizontal nature.
Once a male character in a novel becomes clearly identifiable as the hero, I want him to remain committed to the heroine. He may fight with her and against his attraction for three-fourths of the story, but he may not visit another’s bed. Author Susana Ellis wrote several posts about what she called “Historical Romance Deal Breakers,” and adultery was number two. I concur.
Now, turn the mistress into the heroine … well, I’m all for that. I like a good underdog story.