Michio Sonezaki is a supporting antagonist in the story of Shark Skin Man and Peach Hip Girl. He is the uncle of Toshiko Momojiri and the manager of the mountain hotel where she works. Sonezaki harbors a deep, controlling sexual obsession with his niece, a fixation that drives her to flee the hotel at the start of the narrative. His personality is characterized by a twisted, possessive, and oppressive nature; he is described as sexually abusive, obsessive, and mentally unbalanced. Motivated by his desire to possess and control Toshiko, he refuses to let her go and immediately takes action to retrieve her. Realizing she has escaped, Sonezaki hires a gun-crazed amateur hitman named Yamada to hunt Toshiko down and bring her back, thereby setting a parallel pursuit alongside the yakuza chasing Kuroo Samehada. In the story, Sonezaki serves as one of the primary sources of conflict for Toshiko, representing the oppressive past she is trying to escape. His role is largely static; he does not undergo significant development or change over the course of the narrative. His key relationship is with Toshiko, an uncle-niece bond twisted by his perverse and controlling fixation. He also interacts briefly with the hitman Yamada, whom he hires as a tool for his own ends. Sonezaki possesses no special abilities or combat skills; his threat comes from his obsessive determination and his willingness to employ others to enforce his will. He functions as a mundane yet disturbing figure whose actions set the story in motion and provide a personal, intimate threat to the main female protagonist.