Katsumi Kanzaki is a primary character in the anime 18if, taking on the role of a scientist who studies the nature of the dream world. His academic background defines his approach to the bizarre events of the series, as he approaches the dream realm not as a fantasy but as a phenomenon to be understood and investigated. In the waking world, Kanzaki appears as a man with black hair cut to ear length and black eyes, and is identified as an adult. However, his appearance changes dramatically when he enters the dream world, where he takes the form of an anthropomorphic talking cat. This feline form serves as a visual representation of his role as a guide and observer within the surreal landscapes of others' dreams.

Kanzaki's personality is marked by a serious and analytical demeanor, which makes him an effective counterpoint to the more passive and enigmatic protagonist, Haruto Tsukishiro. He possesses a consistent sense of ethics and is often portrayed as droll or wry, though a few instances show him in moments of over-the-top comic relief that contrast with his usual serious nature. Despite his intellectual and sometimes sarcastic disposition, his actions are driven by deeply personal stakes. His primary motivation for investigating the dream world is not purely academic; his younger sister, Yurina, is a victim of a condition known as Sleeping Beauty Syndrome, having fallen into a coma-like sleep from which she cannot awaken. Kanzaki views Haruto's unique ability to navigate dreams as a critical opportunity to further his research and, ultimately, to find a way to save his sister.

Within the story, Kanzaki serves as an essential ally and source of information for Haruto. While Haruto is trapped in the dream world, Kanzaki can travel relatively freely between dreams and reality, allowing him to provide guidance, context, and occasional warnings about the Witches they encounter. Unlike the many girls afflicted with Sleeping Beauty Syndrome, Kanzaki is a stable and recurring presence across the various episodic dream adventures, acting as a researcher cataloging the rules of this strange dimension. His role extends beyond that of a simple mentor, however, as his own mission regarding his sister runs parallel to Haruto's quest to return to the real world. This personal connection grounds the more fantastical elements of the series, giving Kanzaki a concrete, emotional reason for being involved in such dangerous and unpredictable circumstances.

Kanzaki's key relationships are central to his character development. His partnership with Haruto is the most prominent, as he sees the young man as both a test subject and a colleague in navigating the dangers of the dream world. He also shares a significant bond with a bartender at a bar where he plays guitar, an implied romantic interest that hints at a life and connections outside of his obsessive research. However, his most important relationship is with his comatose sister, Yurina. His efforts to awaken her are the driving force behind much of his work, and the resolution of this personal arc is a major component of his character's journey, though the outcome is described as anticlimactic. Throughout the series, Kanzaki demonstrates notable abilities that make him invaluable. Most critically, he is a dream diver capable of entering the dream world at will, maintaining his consciousness and identity where others become trapped. Inside dreams, his anthropomorphic cat form allows him to interact with the environment and its inhabitants, and he consistently displays a level of knowledge and experience with the dream world's mechanics that exceeds that of the other characters.