Hyōka Fuwa is a first-year student at Tokio Academy and serves as the president of the science club. Her most defining characteristic is a deep, almost obsessive curiosity about sex and the biological mechanisms of human reproduction. She possesses a languid and often flat demeanor, contributing to an appearance that is frequently described as gloomy or tired due to the dark circles under her eyes and her habit of wearing a white lab coat. Her voice is typically monotone, and her wavy black hair has led to her being nicknamed the seaweed woman.

Fuwa’s personality is a blend of a purely logical and scientific mind with a surprisingly caring nature that emerges when others are in trouble. She prefers analytical thinking over emotional discussions and pursues her research with a boldness that routinely places her in socially awkward or inappropriate situations. While she may come across as off-putting due to her relentless questioning, she generally will not act without a person's consent and has shown genuine kindness and a willingness to help her classmates. Her intense dedication to satisfying her intellectual curiosity has led her to take extreme actions, such as staking out obstetrics clinics and directly questioning pregnant women about conception, which ultimately resulted in her being placed on the blacklists of several medical institutions.

Within the story, Fuwa plays a crucial supporting role, often operating behind the scenes to assist the main characters, particularly her classmate Tanukichi Okuma. She approaches Okuma on his first day at school because she believes his transfer from a school with low moral standards means he possesses the knowledge she seeks about how babies are made. She considers him a valuable source of information for her research, and over time, their relationship evolves into a unique and genuine friendship built on mutual understanding. She also has a complicated relationship with student council vice president Ayame Kajou, as they often clash and dislike each other, but they are willing to collaborate when their goals align.

Though not an official member, Fuwa becomes a key unofficial ally of the erotic terrorist group SOX. She provides substantial assistance to their cause of spreading sexual knowledge, acting as a leader and coordinator for ordinary students during obscene events and helping to build public support from the shadows. Her motivations are rooted not in a political ideology but in a scientific pursuit; she seems to view SOX's activities as a means to gather and disseminate the very data she craves. Her personal development is gradual, shifting from a solitary researcher whose methods alienate her to a person who actively uses her intellect to help others. Her attachment to her pet dog, Pes, a male Shiba Inu she rescued, further humanizes her, as she continues to care for him deeply even after his experimental value is gone, stating she cannot die while he is still alive.

Fuwa demonstrates several notable abilities. Her most prominent talent is her exceptional observation and analytical skill, allowing her to quickly realize when a criminal's gun is a fake during a bus hijacking and to discern hidden intentions from a person's subtle movements. She is also highly intelligent and resourceful, ranking first in her grade academically and possessing the technical skill to accurately replicate a discontinued adult device based solely on a verbal description. Her strong will and dedication to her research drive her to take considerable personal risks in the name of science without hesitation.