The Woebegone Wonder Girl Wants to Be Wicked

Despite her hidden genius, Avril has lived a life of misery, constantly belittled as useless by her own family.
When she is forced to substitute for her wicked step-sister in a marriage to a supposedly lecherous, old duke, Avril expects the worst. But surprisingly, Duke Dylan is actually a breathtakingly handsome young man.
He offers her a deal: act as his villainess wife for three years, and then they will divorce. For Avril, this contract is a golden ticket to her ultimate freedom. But as she enthusiastically embraces her new evil persona, her mysterious charm leaves the duke and his household completely bewildered and entirely enchanted.