Mitsuru Yamaguchi is a boy from the town of Kurouzu-cho who attends the same school as Kirie Goshima. He is known among his classmates for his habit of jumping out and startling people, a trait that earns him the nickname Jack in the Box. His background is that of an ordinary student, but his feelings for Kirie begin as a simple crush. As the spiral curse spreads through the town, his crush intensifies into an unhealthy and obsessive fixation. This obsession is further distorted by the curse's influence, which contaminates his mind and behavior. In the narrative, Mitsuru serves as a supporting character whose role becomes prominent in the seventh chapter, Jack in the Box, where his obsession drives him to desperate actions. He is driven by a misguided desire to prove his love for Kirie, leading him to throw himself into traffic and be struck by a car. His key relationship is with Kirie, whom he stalks and fixates upon. He has no supernatural abilities; his only notable trait is his penchant for surprising others and his escalating, spiral‑fueled delusion. His development shows a transformation from a harmless, eccentric classmate into a victim of the curse whose motivations become warped and self‑destructive.