Kazuko Suema is a student at Shinyo Academy whose life was fundamentally shaped by an encounter with a serial killer. During her first year of middle school, she discovered that she had been marked as the next target of a murderer known as Dr. Kisugi. This traumatic revelation had a profound impact, transforming her into a young woman with an intense and unusual fascination with criminal and abnormal psychology. Her desire to understand the motives and methods of such criminals became a driving force in her life, largely fueled by reading the works of an author named Seiichi Kirima. The experience also instilled in her a powerful need to never again be unaware of the events happening around her.

Kazuko’s personality is marked by her sharp intellect, keen observational skills, and her reputation as an exceptionally bright and insightful individual, with Nagi Kirima once referring to her as the smartest person she knows. This intelligence, combined with her vast, often grim knowledge, has earned her the nickname Doctor among her friends. Despite her rationality and calm demeanor, she struggles to fit in with her peers at school, who often view her as strange or morbid due to her detailed discussions of murder methods, which can make others uncomfortable and leave her feeling slightly self-conscious. Nevertheless, she is also a composed, empathetic, and trustworthy person, often serving as a reliable advisor for those who seek her out.

Within the story, Kazuko's primary motivation is to investigate and understand the truth behind the bizarre and supernatural events that plague her city. Her relationship with Nagi Kirima, a classmate who was involved in stopping the serial killer who once targeted her, sparks this interest further. Later, she forms a close and accepting friendship with Touka Miyashita, with whom she studies for university entrance exams. This places her in a unique position as the only character who maintains a deep and trusting friendship with both Kirima and Miyashita. Her other key relationships include her friend Kyoko Kinoshita and later Kotoe Kinukawa, who seeks her help with a relative. A recurring pattern in her role is that she is eager to jump into the middle of incidents but often arrives just after the main conflict has been resolved, causing her to repeatedly miss discovering the full truth.

As the series progresses, Kazuko's development sees her move from a somewhat isolated student to a more central figure. She demonstrates her formidable deductive abilities by winning a psychological battle against Jin Asukai and by quickly comprehending and handling dangerous supernatural situations with impressive composure. In later stories, while still a university student, her potential is recognized by powerful entities, and she is considered as a candidate to become the core of a new unifying organization.

Notable abilities possessed by Kazuko include her extraordinary powers of deduction and psychological analysis, which make her a formidable opponent in mental confrontations. In addition to her natural intellect, she possesses a unique ability known as Moon River. This power allows her to "hide" herself and those nearby in a way that transcends normal scientific or psychological understanding. It is so effective that even superhuman beings like Boogiepop and Walpurgis cannot see through it, though it does not render her invisible to technological devices like satellite cameras.