Yoakemono

The two boys Gin and Jinrou form a tough team in the mountains. One day, Gin decides that the only way they can move forward is to travel to the capital and join the Shinsengumi. It soon becomes clear, though, that they are not tough enough to handle the kind of people they find in the capital. Some of them, including the current Shinsengumi members, smell like beasts to Jinrou, and they seem to have unusual abilities. After Gin's tragic death, Jinrou is determined to become stronger and join the Shinsengumi in his place!

(Source: MangaHelpers)