Samurai Deeper Kyo

At the dawn of the 17th century, at the end of the era of civil wars, in a world of chaos, the epic Battle of Sekigahara was joined. One man emerged from the largest battle ever fought on Japanese soil; a terrible warrior of unspeakable power, he was nicknamed 'the unconquerable.'
Kyoshiro is a peaceful medicine peddler who harbors the soul of an assassin. He accompanies a young bounty hunter across Japan in search of a murderer and on a quest to discover the terrible secret of his own identity. (Source: Tokyopop)