Ouji to Kishi wa Dansou Seijo ga Shinu Hodo Itooshi

The "saint" said to possess "omnipotent power." Because her existence is believed to relieve all suffering, there has been no end to those who seek to misuse her power. Furthermore, the "Saint" is destined to give birth to a royal child, so if she was found, she will be imprisoned in a castle for the rest of her life.

Leif, who was born as such a "saint," has lived a false life as a "man" to protect herself. Since the birthmark that marks her as a saint will disappear when she turns 20, she hides her feelings for her childhood friend, the knight Kai, until one night, when she unintentionally saves a dying man...

(Source: Shogakukan, translated)