Denka ga Hitomebore shita Reijou no Shoutai wa Anata no Goei Kishi desu! the Comic

Elda, a straight-laced knight with zero charm or sex appeal, is suddenly ordered to disguise herself as the noble lady Elriana in order to guard the princess during a grand evening ball. Determined to fulfill her duty as a bodyguard, Elda throws herself into the mission—only for something completely unexpected to happen. Darius, the kingdom's second prince, famous both for his striking good looks and his dislike of women, falls in love with her at first sight...!? (Source: Mag Garden, translated)