Toshiue Elite Onna Kishi ga Boku no Mae de dake Kawaii

Haru, a 16-year-old, is the most prominent prodigy the kingdom has seen in history and has been assigned to the knights of the king. Karen, a 29-year-old single woman, is his boss and the knight running this elite order where the fiercest men in the kingdom gather. Normally, she's a noble woman who's feared as the "Demon Knight Captain," but she's too cute when she invites him to her room, pampers him, and shows a special expression "only to him"! (Source: Kadokawa, translated)