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 the promise he made to protect the kingdom for the rest of his life. When he wakes up again, he finds himself reincarnated as Seretina, the duke's daughter! Even though she's a beautiful girl on the outside, she still has the same determination to become a knight to protect her...