Motoi Yazaki is a supporting character in the anime Ghost Hound, an adult male with short gray hair and black eyes. He is a politician who lives in the town of Suiten and appears to be involved in corrupt activities. His background is deeply tied to the central mystery of the series: as a high school student, Yazaki was friends with Makoto Oogami's parents and with Takahito Komagusu, and together they entered an abandoned hospital. This event is a key link to the supernatural phenomena that later affect the main characters. His past also includes a childhood friendship with Taro Komori, who survived a traumatic kidnapping attempt by a ghost when they were both young.

Yazaki's personality is cold, reserved, and enigmatic. He carries a visible sense of shock and refusal when confronted by Makoto about the fate of Makoto's father, and he avoids answering direct questions about that period. This guarded demeanor suggests deep-seated guilt and regret. He is highly analytical and logical, with a strong preference for working alone and relying on facts rather than emotions. Despite his outward reserve, he is driven by a profound interest in the supernatural and the boundaries of human consciousness.

His motivations center on a need for redemption and for answers regarding his own past actions. He is a scientist who played a significant part in experimenting on supernatural phenomena in Suiten. His research led him to discover Kotodama, a word of power capable of controlling the minds of those who hear it. This discovery reflects his willingness to push beyond ethical limits in pursuit of understanding. Over the course of the story, Yazaki must confront the consequences of his experiments and decisions, leading to a measure of personal development as he faces his past.

Within the story, Yazaki functions as a figure who holds critical knowledge about the events that shaped the protagonists' lives, particularly Makoto's search for the truth about his father. His connection to the abandoned hospital and his experiments with the supernatural place him at the heart of the mystery. Key relationships include his friendship with Makoto's parents and Takahito Komagusu from their school days, as well as his childhood bond with Taro Komori. These ties make him a reluctant but important informant and a source of both answers and further questions for the main characters. His notable ability is his mastery of Kotodama, which he uses to manipulate others, and his deep scientific understanding of the unseen world.