Bakumatsu Yankee

In Edo period Japan, a mysterious foreigner washes upon the shore of Shinsengumi-claimed territory. The foreigner is nicknamed "Yankee," and eventually takes a liking to leader Souji Okita. As the two become good friends and even better warriors, the Shinsengumi faces threats beyond their control, and sends Okita and Yankee to take on enemies to further claim Japan as their own.

(Source: MangaDex)