Youkai no Yome ni Narimashite

For 300 years, the youkai Shirogane has lived alone in the mountains. Until one day he ran into the girl Nadeshiko, whose dream was to study youkai in detail. And the best way to learn about youkai is to get married to one! Before he knew it, Shirogane was tied to Nadeshiko in a contract, the start of married life between an inhuman youkai and a high school girl!