Kiyoko Shimizu is a third-year student at Karasuno High School and serves as one of the managers for the boys' volleyball club. She is known throughout the school for her striking beauty, characterized by her long black hair, a distinct mole near the left corner of her mouth, and the pink-rimmed glasses she typically wears. Despite her aloof and stoic exterior, Kiyoko possesses a deep-seated passion for her role and cares profoundly for the team's wellbeing.

Kiyoko's background reveals a former life as an athlete. During middle school, she was a member of the track and field club, specializing in hurdles. This experience was not a happy one, as she often felt unable to connect with her teammates due to her quiet nature and the large size of the club. She was also prone to injuries, which left scars on her legs that she later chose to hide by wearing tights. Her decision to step away from competitive sports and accept an invitation from her classmate, Daichi Sawamura, to become the volleyball club manager stemmed from a fear of being on the "front lines" as a player. Initially, she viewed the team's battles as separate from herself, providing a sense of relief that she was not directly involved.

However, over time, her perspective shifted dramatically. Kiyoko's primary motivation becomes to fully support the team she has grown to love. She is a quiet but highly capable manager, dedicated to her duties. One of her most pivotal actions is taking the initiative to recruit a new manager, Hitoka Yachi, to ensure the club's future after her own graduation. This act demonstrates her foresight and deep commitment. Her character development is notably tied to Yachi's arrival. Interacting with the anxious first-year allows Kiyoko to open up significantly; she begins to smile more, offers words of encouragement, and even cracks jokes to put Yachi at ease, shedding much of her reserved demeanor. A defining moment in her journey comes when she realizes that she is no longer an observer but an active participant. Watching the team's fierce battles, she declares to herself that even though she does not stand on the court or wear a uniform, she is fighting alongside them on the "front line" as the manager.

In her role as manager, Kiyoko forms several key relationships. She shares a close bond with the other third-year members, Daichi Sawamura, Koshi Sugawara, and Asahi Azumane, who were all present when she first joined the club. She acts as a steady, reassuring presence, whether by retrieving the old team banner to boost morale or by warming Sugawara's cold hands to calm his nerves before a critical match. Her relationship with the underclassmen is also significant. She acts as a protector and mentor to Yachi, and while she calmly endures the loud, daily admiration of Yu Nishinoya and Ryunosuke Tanaka, she does not hesitate to scold them when they become disruptive. It is ultimately with Tanaka that her future lies, as it is revealed she eventually marries him, taking the name Kiyoko Tanaka and working as a clerk at a sporting goods store after graduating from high school.

Despite having no experience playing volleyball, Kiyoko possesses notable abilities derived from her athletic past. She has been evaluated with high marks in speed, jumping, and stamina, reflecting her track and field training. This foundation gives her quick reflexes, allowing her to effortlessly stop a stray volleyball from hitting Yachi. She is also capable of performing an accurate underhand serve. Kiyoko Shimizu transforms from a quiet observer who viewed sports from a distance to a dedicated, quietly forceful, and indispensable pillar of the Karasuno volleyball team.