Kuenai Kishi-sama no Amaiki Seikatsu Shinan: Kimi no Iitokoro, Boku ga Zenbu Oshieteageru

Camilla, vice-captain of the all-female knight order known as the Thorn Company, has a constant source of irritation: Frederick, the captain of the Second Knight Order and her childhood friend turned inseparable nuisance. He still treats her like a little sister and always stands far too close for comfort. One day, Camilla catches a delinquent soldier harassing a maid and sharply reprimands him. The humiliated soldier challenges her to a duel, which turns out to be a drinking contest. Just then, Frederick appears out of nowhere and declares, "Let me join in," throwing himself into the competition. Camilla, a heavy drinker, easily defeats the soldier, but somehow ends up in a one-on-one match with Frederick at his home. The next morning, Camilla wakes up in Frederick's arms.

(Source: Bunkasha, translated)