Kamen Hakushaku wa Kurosuishou no Hanayome ni Koi wo Suru

Also known as: 仮面伯爵は黒水晶の花嫁に恋をする

by Ren Hidou, Kei Kozakura

Christa was sold off by her scheming stepmother in an arranged marriage to Lord Gerald, a notorious misanthrope who always wears a mask. Gerald tells Christa coldly, "It is not a wife I need, but an heir." But Gerald is actually hiding a secret that he cannot tell anyone...? (Source: FK Comics, translated)