Currently thinking a kingdom building plot. My MC is the son of a noble and at first, he just wants to take it easy and live a life of luxury. Until a monster attack ends up killing his parents, forcing him to step up and rule the territory.

One of his harem would be his fiancee. She is a sickly and lazy girl. Weak since birth, she spends most of her time in her room, sleeping or reading fantasy rooms. Her illness sours her disposition, making her apathetic to everything. She has no dreams or desires. She just lives day by day, waiting until the day her illness claims her.
So, when her father, who despises her for being a NEET, gives her away to be MC's fiancee, she doesn't even bother to pretend to be a proper lady, openly declaring that she doesn't like him and she doesn't want to do any work. And if he wants to have sex, he's the one who has to do all the work. And any kids they will have, she won't raise them.
However, MC, who's also lazy, agrees with her lifestyle. The two ends up spending her visiting days in his room, reading fantasy books and playing board games. For the first time, she has a true friend, and that was enough for her to actually be willing to be his wife.
The truth is, her illness comes from her body producing too much mana. She has the potential to be a powerful sorceress.
When MC's territory gets destroyed by monsters, she quickly comes to his side. Sleeping together every day, she helps him go through the death of his parents. And she gives advice on how to run his territory as well, thanks to her books having stories of young lords managing their territories.