The Runaway Extra and the Obsessive Duke

Story

In this world, a duke's full power can only be unlocked by uniting with his "phero," his destined mate. So, to undermine his rival, Krua's father sends her to Duke Arzenta disguised as his phero, hoping she'll take his innocence and his power with it. But Krua has no intention of playing along! If she can just convince her fiancé to join hands with her and keep up the enamored newlyweds act until his real phero appears, she'll be able to flee safely when the time comes... But will he let her go?

Synopsis via AniList (English).