Dekasegi Reijou no Konyaku Soudou: Jiki Koushaku-sama wa Konyakusha ni Aisaretakute Hisshi desu.

One day, Irina, a poor countess, is asked to marry a man! She has resigned herself to marrying for the sake of her family, but there is one thing she will not compromise on. That is to be useful to the next Duke Michal, whom she has admired since she was a child, and whom she worships as "god." Realizing that marriage would leave her unable to move for him, she sets off to work in Michal's mansion! Without hearing the most important part: that her fiancé is Michal... A romantic comedy about a girl who works at her fiancé's mansion without knowing who she's engaged to, and a young man who struggles to be seen as her fiancé.

(Source: Ichijinsha, translated)