Shin no Nakama Episode.0: Imadake Saikyou no Souryuu Kishi wa, Izure Musou no Imouto Yuusha wo Mamori Nuku

Born a swordsman of overwhelmingly high skill, Gideon Ragnason is the older brother of the ‘Hero’. He spent his days honing his skills to protect his sister, who would one day save humanity. Then, the day of calamity finally arrived. Their homeland was assaulted by the Demon King's army. Gideon, now a knight without equal, revealed to the villagers that his sister, Ruti, was the destined Hero, in order to inspire them. “We are the army of the ‘Hero’! Therefore, what we direct at our enemies should not be fear, but the wrath of retribution!” The destinies of the genius knight dancing through swordplay and the girl awakening to the power of the Hero now quietly begin to move.

(Source: Kadokawa, translated)