The Knight Captain is the New Princess-to-Be Vol. 1 - Yasuko Yamaru

In this shojo rom-com tale, a dashing female bodyguard pretends to be engaged to her childhood friend, the prince...but is their fake betrothal really just an act to him? Christina, a.k.a. "Lady Chris," was born into a noble family and treated more or less like a boy growing up. Now a dashing young woman, Chris is not only captain of the imperial guards–she personally protects Prince Leonardo, who has been a dear friend since childhood. When his father, the king, demands he find a suitable girl to marry, Leo insists that he's already found one: Chris! Chris is shocked, but figures that Leo doesn't really love her like that; it's probably just some ploy to keep the king happy. Chris decides to play along, but as the charade goes on, she starts to wonder if maybe her princely pal has actually fallen for her!