A Happy Marriage with the Switched Young Lady

I didn't do anything special, yet I suddenly got transported to another world. When I arrived, there was a beautiful girl who wasn't breathing anymore... In exchange for being guaranteed clothing, food, and shelter, the protagonist agrees to become Lillia to protect the territory. I was told I could do whatever I wanted as long as I didn't leave the mansion, but what? Suddenly married?! This is a mismatched romantic comedy story between a Duke who prioritizes his duties and has an expressionless face, and a carefree young lady (fake) who does things at her own pace.