The manga's main antagonist, Kazuo Kiriyama, is a ruthless killer with no remorse. He possesses a genius-level IQ, allowing him to master skills like playing the violin in minutes, and he adapts quickly to various situations.
Kazuo's starting weapon is an Ingram MAC-10 Uzi. His blood type is AB. The characters in his given name mean "harmony" and "masculinity," while those in his family name signify "paulownia" and "mountain."
In both the novel and manga, it is implied that Kazuo was born out of wedlock due to his mother's infidelity. After killing his gang in the novel, he touches his left temple, experiencing an elusive feeling. Kazuo mentions that since the doctor who operated on him died and his "real father" passed away from complications, he has no one to question about this.
In the manga, Kazuo mirrors the novel's depiction, mercilessly killing his classmates without hesitation or regret. Some fans note physical similarities between Kazuo and Hiroki. Both are nearly the same height and resemble each other as children and teenagers. They also acquire the same eye injury and are highly trained martial artists, though Kazuo has the upper hand in combat.
A notable difference from the novel is that Kazuo's brain damage results from a childhood car crash rather than in utero. This event is depicted in volume 14 when Kazuo, wounded by Noriko Nakagawa, flashes back to his happier years. It is hinted that Kazuo may be colorblind when he expresses shock at colors and wishes for everything to "go back to grey." However, this conflicts with his earlier observation that "They all bleed red."
In the manga, Kazuo is portrayed as a near-perfect genius. From childhood, he displays incredible talents and wide-ranging knowledge as a junior high student. He reads books on human anatomy and martial arts and quickly grasps new concepts. For example, he learns Judo during a martial arts lesson to defeat his teacher, even pulling out his eye out of curiosity about the liquid inside it.
Kazuo is more expressive of his feelings and curiosity in the manga. He engages in longer conversations, such as with Mitsuru in volume 2 and Hiroki in volume 12. After vomiting from stress, he tells Hiroki about how his father's chauffeur killed someone in a car accident and displayed symptoms similar to Hiroki's ulcer. From then on, he barely speaks again until being shot by Noriko Nakagawa, whispering "Shuuya" as he dies. This death hints at a return to normality for Kazuo, as he whispers "Shuu" beforehand, implying a care for Shuuya. In the English version, he says, "Shuu... I can feel again."
Kazuo kills 14 people in the manga and ends up in 4th place.
