Izumi Koyama is a prosecutor working for the Public Prosecutor's Office, specifically within the Joint Coordinating Department. This organization was established in post-war Tokyo under pressure from foreign powers to regulate and control various government bodies. In her role, Koyama holds significant authority, including the power to use her own judgment on whether to launch an investigation, which frequently draws her into cases with considerable social impact, including those involving the protagonist Shinjuro Yuki.

In terms of appearance, Koyama is a tall, slender woman with dark blue eyes and black hair. Her hair is parted on the right, with the right side reaching her shoulder while the left side is cut shorter, around mouth-length. She typically wears a dark green suit consisting of a vest buttoned over a white shirt, paired with black shoes.

Koyama possesses a generally no-nonsense personality and presents a calm, professional exterior. However, beneath this composed surface, she has a softer, more vulnerable interior, and her flaws become subtly apparent at times. Her primary motivation is a strong admiration for Rinroku Kaisho, the influential chairman of the JJ System, whose work she respects for promoting social stability. She has a personal crush on Kaisho, which deeply influences her actions and allegiances. Due to her loyalty to Kaisho, she harbors a dislike for Shinjuro Yuki and his companion, Inga, though her position often prevents her from openly expressing this animosity. Her relationship with others is complex; for instance, during the theatrical film adaptation of the series, her softer side is more prominently displayed, showing her as a genuinely kind person who initially got along well with Inga when the latter appeared as a young boy.

Throughout the story, Koyama serves as a representative of the established authority and often finds herself in opposition to the detective Shinjuro Yuki. Her character undergoes notable development, particularly when her loyalties are challenged. In the latter half of the series, when Inga reveals Kaisho's secret publicly, Koyama falls into a state of panic. Following a subsequent explosion accident in which Kaisho is presumed dead, she is depicted as the person who mourns his loss the most deeply at his funeral. Regarding notable abilities, Koyama does not possess any supernatural or investigative powers; her primary capability stems from her official authority as a prosecutor who can determine whether an investigation proceeds, making her a pivotal figure in the justice system of her world.