Kazuko Suema is a student at Shinyo Academy whose life is defined by a traumatic encounter from her past. As a first-year middle school student, she was marked as the next target of a serial killer. Although she was saved before any harm came to her, the psychological impact of being chosen as a victim was profound. This experience instilled in her a deep-seated need to understand the minds of those who commit terrible acts, leading her to develop an intense and unusual interest in criminal and abnormal psychology.

This fascination shapes much of her personality and her interactions with others. She possesses an extensive and often morbid knowledge of true crime, including specific murder cases and methods. While her classmates perceive her as a normal, if somewhat reserved, girl, those who get to know her soon discover her grim hobby. She is acutely aware that this interest makes her seem strange, and as a result, she often feels like an outcast, struggling to find acceptance among her peers who tend to exclude her for being weird. Despite this, she is a kind and fundamentally nice person at heart, willing to offer advice and encouragement to those who are suffering, even if she downplays the value of her own words.

Her primary motivation is a refusal to be left in the dark again. The trauma of nearly being killed without her knowledge drove her to seek out and understand the truth behind the strange occurrences in her city. She does not want any event to pass her by without her being aware of it. This drive leads her to investigate rumors and anomalies, though she is also a pragmatist who is skeptical of things she cannot see for herself. She initially dismisses the fantastical rumors surrounding Boogiepop, preferring to rely on concrete evidence. Her quest for understanding often puts her in the orbit of other key individuals, particularly Nagi Kirima, whose involvement in stopping the serial killer years ago fascinates and inspires Suema.

In the narrative, Kazuko serves as a recurring point of view character who is deeply curious about the supernatural but constantly finds herself just a step behind the most crucial events. She has a knack for arriving after a situation has already been resolved, making her a witness who pieces together the aftermath rather than a direct participant in the climax. Her role is often that of an investigator or an advisor; her sharp insight and vast knowledge earn her the nickname Professor among her friends, who trust her judgment and come to her for help. For instance, she is approached by Kotoe Kinukawa to investigate the mysterious Jin Asukai. Her analytical abilities are formidable, enabling her to win psychological battles against formidable opponents and deduce critical information where others cannot.

Her key relationships place her in a unique position within the story's social web. She is one of the few characters who is close to both Nagi Kirima and Touka Miyashita, forming a bridge between two of the most pivotal figures. Her friendship with Touka is particularly significant; she confides her secret interest in crime to Touka, trusting her completely, and the two spend a great deal of time together, often studying. Because of this closeness, she is frequently present when Boogiepop manifests in Touka's body, and while she can sense that something is different about her friend, she never deduces the truth of Boogiepop's identity. She also forms a close friendship with another student named Miyashita, focusing on their studies for university entrance exams. Her circle also includes Kei Niitoki, with whom she is on friendly terms.

Kazuko undergoes a notable development from a diligent student with a dark hobby to a figure of significant power and influence. While she starts as an ordinary, if brilliant, human, later events reveal her to be an MPLS, a being with special abilities. Her specific power is called Moon River, which allows her to hide herself or other people nearby from detection. This ability operates on principles that defy conventional science and even confounds other supernatural entities, making her a valuable and formidable presence. In later storylines, she progresses to university, where her strength and judgment impress powerful beings like Oxygen, who ultimately selects her to become the new Axis, or a central leader, for the Towa Organization, a significant step up from her previous role as an outsider looking in.