Kokugetsusai, also known as the Phoenix Killer or Phoenix Slayer, is a legendary martial sage who served as the master of Jinka Yamato and founded the technique of spirit transformation. He had a history of repeatedly challenging the goddess Ōyama Mitsuchihime and being defeated each time, a process that tempered his strength and sharpened his abilities. Kokugetsusai died before the events of part one of the story and is initially only referenced in conversations. Eight years later, during part two, he returns as a ghost by possessing the character Taizan and appears before Tsukiko. As a ghost, he grants Tsukiko a gourd that possesses land‑draining powers similar to those of the Thousand‑Fox Spirit, and after Taizan becomes bound to the land, Kokugetsusai transfers to a spiritual fragment of Hanatora. His personality is marked by a relentless drive for self‑improvement; his willingness to face a god repeatedly and his creation of a major martial technique demonstrate a determined, growth‑oriented mindset. His motivations revolve around the pursuit of greater strength and the development of spiritual combat methods. In the story, he functions first as an influential deceased mentor whose teachings shape Jinka’s path, then as a ghostly figure who directly aids Tsukiko and interacts with other characters. His key relationships include his student Jinka, whom he trained in spirit transformation, and Tsukiko, to whom he entrusts the gourd. His notable abilities include mastery of spirit transformation, the combat prowess that earned him the epithet of phoenix‑slayer, and, in his ghostly state, the use of the gourd’s land‑draining power.