Formerly, the Fallen Daughter of the Duke: Volume 3 (Light Novel) by Ichibu Saki

A newfound harmony flourishes between the kingdoms of Noston and Paffuto, and beyond international relations, fresh opportunities for friendships and romance arise! As their relationship deepens, Vik wants to help Claire face her scheming half-sister, Charlotte, at the upcoming ball. Although there’s no save point before this boss encounter, Claire is now equipped with a mysterious arcane treasure that’s always been surprisingly close at hand and also has a party of friends to support her. They just might put a stop to Charlotte’s furor...but the younger Martino daughter is not afraid to resort to underhanded tricks!