Mirai ni Tobasareta Kensei, Nakama no Shison wo Mamorutame Musou Suru

Isamu, a member of the hero's party known as the "Sword Saint", was flung 200 years into the future as he tried to protect his friends from the magical powers released during the final battle with Demon Lord. Lost with nowhere to go, Isamu is saved by a girl named Cecilia, the descendant of the original hero. Following the prophesy passed down by her ancestors, she takes in Isamu and promises to take care of him. In return, Isamu decides protect Cecilia on behalf of his former comrades, but…!?

(Source: Brave Bunko, translated)

Note: A Silver Prize winner in the 2nd Hifumi Shobo Webnovel Awards.