The last boss from Exclusive Underground -Dark Night- (閉鎖区 Underground -Dark Night-/Heisaku Underground -Dark Night-), and the ex-captain of a military outpost of Team STRAY dogs. He used to be the best friend of Shou, the hero of the story, but he murdered their friend Rokkaku Araki and Shou's sister, Kokonoka, and deserted the team. He established the association Arajin and conquered Sacred Tree Kuronagimaru and the spirit Hangaku. In order to gain control of the territory of Adachi ward, which had turned into a lawful land after a huge natural disaster, he fights against Shou.

Yuya is a violent and aggressive person who enjoys picking fights, though he is shown to have a sense of honor. He is not interested in making his creator alter his world, being fine with the way he is. When confronting Mamika Kirameki he states that he hates dealing with girls who think that they can get away with things by crying. Yuya is also kind of cocky warning Mamika that he is strong and in his world apparently he is the final boss.