He's Just My Brother, Your Grace!

In a fantasy novel, I suddenly became Astel, the younger sister who never appeared. While my brother Cassian is fighting for revenge of the clan, it is my duty to find and deal with the final villain that remains unrevealed even at the end of the series! In order to do that, I must first enter the mansion of the northern duke, where the traitors of the family are hiding. But somehow, I became the 'mate' of duke Anais, a beast and a prime candidate for the ultimate villain. Besides... "Kiss me now, Astel." In order to live, I have to communicate with my 'mate' every day, but there’s no such thing in the original! "Shouldn’t we go see the ocean you like today?" "I-I don’t think I can. I have to go see Mr. Cassian today." The duke gritted his teeth with a cold face. I was a little scared...