Gakeppuchi Reijou wa Kurokishi-sama wo Horesasetai!

Clarice's younger sister is her whole world. In a world of monsters, born into a noble family that's gone broke, there's nothing Clarice wouldn't do for her. So when she gets sick, and there's only one way to save her, Clarice doesn't blink! She agrees to an arranged marriage with the Dark Knight, a ferocious warrior who goes everywhere in magical armor—even their wedding! Afterward, Clarice is ready to consummate the marriage and charm her new spouse into paying to save her sister's life. There's just one problem: The Dark Knight won't let her into the bedroom! And Clarice finally finds out why...she's secretly a woman! With female knights forbidden, the Knight seriously lacking in social graces, a household full of servants scared out of their wits, and monster fights breaking out constantly, Clarice realizes that this wedding of convenience is going to be more inconvenient than she bargained for!

(Source: Kodansha USA)