Byakka Ryouran: Shiroki Shoujo to Tensai Gunshi

The manga is about a girl named Eidai Shuku, who searches for missing people, while a great war is approaching. During her travels, she meets a young man who is not skilled in horse riding or martial arts, but becomes extraordinarily strong during a battle.

(Source: Anime News Network)

Note: Adapted from Yoshiki Tanaka's 1998 novel Honryuu.