Kurokami no Lady to Koi no Jikenbo

From Renta: In the nineteenth century, Kerry pretends to be a man and helps the powerless as a detective in London, though she's from an aristocratic family. She takes an assignment involving a stolen diamond for a couple who's facing their parents' objection. The man standing in front of her is Hohen, a family lawyer with a sharp stare. They're at odds with each other at first but as they cooperate to solve the case, they become attracted to each other.