Koutei Heika no Scandal Baby: Toubousuru Hazu ga Amaku Tsukamaeraremashita

Under her father the margrave's command, Lewis Ballantine is enlisted in the Monastery Knights to perfect her swordsmanship skills. While it's common knowledge to most of the swordsmen that Lewis is indeed a woman, crown prince and heir to the Flamelance throne, Gerald, is blind to this fact. The night of the ball for knights, Gerald appears suddenly in Lewis's room. While surprise visits are commonplace, Gerald is overtaken by this figure of Lewis he's seen for the first time?! Source: MangaPlaza