Demon Eyes Kyou, whose real name is Mibu Kyou, is a legendary man, known for his red, demon-like eyes, and called the "Thousand Slayer" for killing over one thousand samurai during the Battle of Sekigahara before being defeated by a young man named Mibu Kyoshiro. After being defeated, his mind fused with Kyoshiro, and his body was hidden away somewhere in the forest of Aokigahara. He is very sadistic and enjoys slaughtering anyone. Over time, his blood lust slowly decreases, though he still kills without mercy. It continues to peak at times, though training with Muramasa helps fully calm him. During this time period when he first meets Yuya, he finds himself very protective over her, though handling it in a very childish manner. Such as calling her "dog face" and "ugly." Though it is stated by Kyoshiro, that he indeed likes Yuya.
Kyou's weapon is Tenro, one of the "Four Great Demon Blades." It was forged by Muramasa, and it is nearly unbreakable. His sword style is The Mumyou Jinpuu Ryuu Satsujin Ken, which was taught to him by Muramasa. It teaches standard fencing skills and defense, but they are far more powerful than any of the skills of other sword styles. It is the most powerful sword school in the Mibu Clan. It utilizes attacks that can cut air and create illusions. He later learns a higher form of the style that utilizes the Four Symbols (Genbu, Seiryuu, Byakko, Suzaku and even Kouryuu), manifesting a number of different powers based upon the creatures.
Kyou is the last remaining True Mibu and has the True Red eyes, which allows him to turn into a Battle God or Demon God. After he turns into the Battle God and suppresses his battle lust, he is granted the mark, and thus the title of the Aka no Ou. Kyou was born in the Mibu village and was friends with the former Aka no Ou. Once the Aka no Ou turned evil, Muramasa took Kyou from the village so that he may grow up to be the 'hope' of the Mibu clan. Kyou travelled the world and formed the Shiseiten in order to become strong enough to return to the Mibu village and defeat the former Aka no Ou. By the end of the series, he is acknowledged as the strongest fighter in the series. In the bonus chapter of the manga Yuya and Kyou live together.