A Inherent Drop
Due to his actions in the novel, he can be considered as the main antagonist.
Kyougoku Natsuhiko appears as a white-haired man, dressed in torn traditional kimono set. He dons a red mask on his head (possibly symbolizing his role as the 'demon').
Kyougoku's ability, A Inherent Drop (憑き物落とし, Tsukimono-Otoshi?), or Exorcism allows Kyougoku to give someone a 'demon' which then alters their mind/spirit.