Ribbon no Kishi (1967)

Princess Knight

In the kingdom of Silverland, Prince Sapphire holds a big secret: he is actually a girl. When Sapphire was born, she accidentally received both the blue heart of a boy and the pink heart of a girl. To protect the kingdom from the evil Duke Duralumin, who is the successor to the throne, Sapphire leads a double life, posing as a boy as women are not allowed to reign. God assigns trainee angel Tink to retrieve Sapphire's boy heart, but she refuses to let go of it. As the Duke schemes to usurp the throne, Sapphire finds herself in an even worse predicament in the clash of her two identities—she falls for Prince Franz Charming from the neighboring kingdom. Sapphire and Tink now embark on exciting adventures, but along the way, they must fight various enemies to uphold the throne at all costs. [Written by MAL Rewrite]