Samurai Deeper Kyo

It is 1604, four years after the Battle of Sekigahara, perhaps Japan's greatest civil war. The people try to recover from its aftermath and enjoy the newly attained peace. Amid such times Shiina Yuuya, a roughneck girl who earns money by capturing outlaws, mistakes a roaming and peace-loving medicine seller Mibu Kyoushiro for the legendary wanted man "Onime-no-Kyo." But she soon realizes that she is wrong when she sees that Kyoushiro is just a weakling. Nevertheless, she ends up capturing Kyoushiro anyway -- he is wanted for running off without paying for a meal. As they travel together Kyoushiro tries to help Yuuya catch another wanted criminal. In the process Kyoushiro is cornered. Then he mumbles, "Here he comes," and the color of his eyes turn a deep red. He pulls out his sword which he had never touched before and suddenly, Onime-no-Kyo, the legendary assassin said to have killed one thousand men with this sword, is standing there. Kyoushiro and Kyou: Two spirits in one body. Yuuya is looking for a "man with a wound on his back" who killed her older brother. The two head to Edo together. One after another, samurai come after Onime-no-Kyo. Why do Kyoushiro and Kyou share the same body?