Izumi Koyama is a public prosecutor with the authority to initiate investigations. She brings Shinjūrō Yūki and Inga to the Bettenou followers' gathering place, facilitating their meeting with Seigen Hayami, who explains the circumstances of the followers' deaths and the involvement of doubting family members. Koyama is frequently involved in cases investigated by Shinjūrō Yūki. After Shinjūrō is accused of murdering Myoushin Oono, she terminates his previous identity and provides him with a new one: Shinjūrō Yūki. Her professional role consistently places her within investigative contexts, intersecting with the activities of Shinjūrō and Inga.