Kei Niitoki is a student at Shinyou Academy and serves as the president of the school's disciplinary committee. She is a second-year high school student who, despite her age, has a youthful appearance that often leads others to mistake her for a middle school student or someone even younger. This contrast between her small stature and her position of responsibility is a notable aspect of her daily life at school. She has shoulder-length black hair and brown eyes.

Known for her straightforward and sharp-witted nature, Kei possesses a strong sense of duty that she feels almost as a compulsion. Once she identifies a problem, she feels an inherent need to solve it, which is the primary reason she took on the role of discipline committee president. Her peers see her as reliable and mature, often seeking her advice and treating her with the respect one would offer an older sister, despite her youthful looks. Kei herself finds her tendency to meddle in other people's affairs to be somewhat troublesome, recognizing it as an internal drive rather than a noble pursuit.

Kei holds a very special and rare position within her world as one of the few individuals who knows the truth about her classmate, Touka Miyashita, specifically the connection between Touka and the entity known as Boogiepop. This knowledge complicates her personal feelings, as she harbors a romantic crush on Keiji Takeda, who is Touka's boyfriend. She is aware of the deep bond between Touka and Keiji and comes to understand her place in that dynamic, leading to a strained but resigned relationship with Touka. Her feelings for Keiji are something she eventually confronts and chooses to move past.

Her role in the story often puts her in the center of supernatural events. Kei’s most prominent ability is her exceptional insight and sharp observational skills. She is able to see through contradictions and manipulations that confuse others, such as when she perceives the internal conflicts of the King of Distortion. Her quick thinking and strong will allow her to navigate dangerous situations, including the incident known as the Manticore Shock, and she is considered one of the people who managed to endure that crisis. She has proven capable of standing her ground against powerful adversaries like Kishimori Yuhki and has worked directly alongside Boogiepop, even making her way to the heart of a confrontation with the King of Distortion inside the Moon Temple. Throughout these ordeals, she demonstrates a quiet strength that allows her to grow and change her perspective on her classmates, including Touka Miyashita and Nagi Kirima, with whom she becomes closer after sharing these extraordinary experiences.