Norio Katayama is a classmate of Kirie Goshima at Kurôzu-Cho High School. He is a peculiar student who only attends school on rainy days, a habit that marks him as lazy and unmotivated. Katayama is noticeably overweight and moves at an extremely slow pace, both physically and in terms of reaction time. He often appears damp from rain or snail slime and has a sluggish, almost absent demeanor. This unusual behavior makes him an easy target for bullying, particularly by Tsumura and his friends, who mock his labored movements.

Katayama’s role in the story is that of an early casualty of the spiral curse. He is the first person in the town to physically transform, gradually mutating into a snail-like creature. His slow nature is mirrored in his transformation, which unfolds without resistance or awareness. The change begins with a spiral pattern appearing on his back, and over time his body becomes more slimy and snail-like, losing human characteristics. His progression serves as a disturbing preview of how the curse can overtake individuals, especially those who are already passive or withdrawn. Katayama does not develop further as a character; his transformation is complete and he becomes a part of the town’s growing spiral-afflicted population. He has no notable abilities beyond his preternatural slowness, which ironically makes him a fitting vessel for the curse’s insidious effects.