Kazuyuki Izumi is the father of the protagonist, Shinichi Izumi, and the husband of Nobuko Izumi. He is a 44-year-old man who works as a freelance writer, having previously been employed by a magazine. Physically, he is a tall, middle-aged man with black hair, which he keeps neatly combed, and a distinctive mustache. His facial features are notably similar to those he passed on to his son, Shinichi. In the anime adaptation, he is depicted as a modern, tech-savvy individual, often seen reading news on a tablet and using a cellphone, reflecting an ability to adapt to new technology.

At the beginning of the story, Kazuyuki has a relaxed and somewhat laid-back personality. He is generally easygoing and tends to go with the flow, even as he notices subtle changes in his son’s behavior and physical abilities after Shinichi merges with the parasyte Migi. He is not above lightheartedly teasing Shinichi about his childhood fear of bugs. However, his life is shattered when he and his wife are attacked by a parasyte during a trip to Izu. While Kazuyuki survives the attack with injuries that require hospitalization, Nobuko is killed and her body is taken over by the parasyte.

This traumatic event fundamentally changes Kazuyuki. Overwhelmed by grief and the horrifying memory of seeing his wife transformed into a monster, he becomes deeply depressed and develops a drinking problem. His relationship with Shinichi becomes strained and complex. Initially, he resents his son’s seeming ability to move past his mother's death with an unnerving level of composure, which is in fact a side effect of Shinichi’s own trauma and his growing detachment from his humanity. In his anguish, Kazuyuki lashes out at Shinichi with harsh words. He suspects that something is profoundly wrong with his son, living in a state of denial mixed with a fearful suspicion that Shinichi may have also been replaced. Despite this, his paternal love ultimately prevails, and he eventually reconciles with Shinichi, opening up about the terrible fate that befell Nobuko.

Kazuyuki’s primary role in the narrative is that of a tragic witness and an anchor to Shinichi’s remaining humanity. He does not participate in the physical battles against other parasites. Instead, he represents the ordinary human cost of the parasitic invasion. His presence and his suffering serve as a constant reminder to Shinichi of what he has lost and what he is fighting to protect. As the last remaining member of Shinichi’s immediate family, Kazuyuki symbolizes the fragile structure of a normal home life that has been irrevocably broken. For Shinichi, his father is an emotional burden, but also the final link to his former human life, forcing him to hold onto his empathy and not completely succumb to the cold, logical nature of Migi.

Kazuyuki’s development is a journey from a carefree father to a man broken by post-traumatic stress, and finally toward a slow process of reconstruction. In the epilogue of the story, he is no longer the shattered man he was in the immediate aftermath of the tragedy. He has chosen to look toward the future and accepts his son as he is, without pressing for answers about the lingering shadows of the past. He embodies the human capacity to adapt and survive even the most extreme psychological trauma. While he is not a hero of action, he is a figure of quiet emotional resilience, providing a measure of stability and a testament to the idea that even a devastated family can, with time, find a way to continue.

Kazuyuki does not possess any notable supernatural or combat abilities. His strengths are entirely ordinary and human, rooted in his roles as a father and husband. He is a civilian whose world is defined by domestic life and his work as a writer, making him completely vulnerable to the horrors introduced by the parasites. His only "ability," if it can be called one, is his deep, if sometimes faltering, capacity for paternal love and his ultimate resilience in the face of unthinkable loss.