Imouto ga Onna Kishi Gakuen ni Nyuugaku Shitara Nazeka Kyuukoku no Eiyuu ni Narimashita. Boku ga.

A younger sister attends a prestigious all-female knight academy and performs exceptionally well in all areas. She claims her older brother is far stronger than she is, despite his belief that he is average. The academy's top knight takes issue with this and issues a challenge. During a sparring match, the knight is quickly outmatched. The brother's simple actions, such as giving a post-battle massage, draw unexpected admiration. His ordinary demeanor leads to attention from the princess and other female knights, despite his unawareness of his own abilities.