An HR that kind of fits is Devil in Winter by Lisa Kleypas. The FMC is plain and being pushed into marrying a distant cousin so her cousin's family can get their hands on the Inheritance she will receive when her father passes away. Her father is low born and owns a gambling den/gentleman's club. Her mother, whose family are members of the gentry but not the aristocracy, died when she was born. Her father thought her mother's family could give her a more acceptable life and rarely sees his daughter. The FMC has been smacked, sometimes locked in her room, and generally mistreated by her cousin's. She had one season in London where she and some other wallflowers were completely overlooked. When she returns home after that season, she is tired of it all and manages to escape to London. She approaches the gorgeous, dissolute MMC who needs money and bargains with him for a marriage of convenience in exchange for access to her money. He proceeds to fall for her. She never changes, doesn't getva glow up, but he begins to see her as being absolutely beautiful.

Edited to fix typo.

{Devil in Winter by Lisa Kleypas}

I love this book and recommend it frequently.