Kokū (虚空) is one of the creators of madōgu, and a Hokage Flame Master and leader of the Flame Dragons before Ōka. He and his rival Kaima competed to create madōgu, but he is forced to kill his rival once the latter succumbed into further madness.
He is often portrayed as perverted but also very wise. As one of the creators of the madōgu, Kokū is knowledgeable about the many different types of powers each madōgu possess including ones that his rival Kaima created such as the Tendō Jigoku. He appears to possess great insight as he realized that Kurei was far stronger than Recca and opted to train him on how to properly utilize each of the flame dragons' powers.
AbilitiesAmong all the flame dragons, Kokū is the only one that is capable of manifesting his human form outside of Recca's body. the most unique. Kokū's power takes the form of a giant, concentrated laser beam. He has a mirror that allows a person to enter their mindscape. As one of the original creators of madōgu, Kokū knows a great deal about each madōgu's strengths and weaknesses.
- His physical appearance as well as characteristics resemble that of Master Roshi from the Dragon Ball series.