Onime no Kyo is said to have slaughtered a thousand men. Although his reputation actually belongs to Kyoshiro, he has certainly killed more than a thousand people himself. He is imprisoned in Kyoshiro's body and takes over whenever Kyoshiro's life is threatened. Kyo fears no one and is quite arrogant and self-centered early in the manga. Throughout the story, he is looking for his real body, which was sealed somewhere by Kyoshiro. His ultimate goal is to become the strongest man in the world. His most distinguishing feature is his red eyes.
He seems to harbor feelings for Yuya as seen in the anime and also in the manga. In Chapter 235 (Volume 30) of the manga, he states that he will not let Kyoshiro take his body nor the girl (Yuya). It is also seen in the bonus chapter that they live together and are lovers.