Ken to Tiara to High Heels: Koushaku Reijou ni wa Eiyuu no Tamashii ga Yadoru @comic

The kingdom's hero, Ortus (♂), dies in battle after failing to fulfill a promise he made to protect the kingdom for the rest of his life. When he wakes up, he finds that he has been reincarnated as Seretina (♀), the duke's daughter! Even though she's a lovely, beautiful girl, she's still the same and determined to become a knight to protect her country in this life. However, besides being sickly and fragile, she is also faced with the harsh fate of a lady. She has to learn manners from her horrible mother, marry a man she doesn't want to marry, and many other challenges that will keep her from becoming a knight. Then, on the way to the royal castle, a demon attack reveals her true power that no one knows about! A new heroine is born, with the strength to slay demons with a single blow, and the peerless beauty to captivate even people in high society! She's pretty on the outside, but fearless on the inside! The chivalric, heroic fantasy of a feeble young woman begins! (Source: TO Books, translated)