Noriko Sonozaki is a central figure in the Kiznaiver narrative, introduced as a mysterious and seemingly emotionless high school girl who orchestrates the Kiznaiver experiment. She was once a participant in the original Kizuna Experiment as a young child, during which she became permanently linked to Katsuhira Agata. In that experiment, she absorbed the overwhelming physical and emotional pain of eighteen other subjects, leaving her burdened and her emotional responses numbed. To cope, she has been taking medication that suppresses her ability to feel pain and emotions, resulting in a cold, detached, and authoritative demeanor that makes her difficult to read.

Her primary motivation is to prove the Kiznaiver system can create genuine understanding among people by forcing them to share each other’s suffering, with the ultimate goal of achieving world peace. On a personal level, she longs to reconnect with Katsuhira and to rediscover the feelings she lost, hoping that the experiment’s success will allow her to “find herself within him.” Despite her clinical approach, she occasionally reveals glimpses of vulnerability, such as when she admits she does not fully understand the emotional intricacies of the Kiznaivers or when she asks Katsuhira directly about his feelings for her.

In the story, Noriko acts as the architect and overseer of the renewed Kiznaiver program. She personally recruits six of the seven Kiznaivers and tasks them with finding the seventh member on their own. She designs tests and challenges to push the group toward bonding through shared pain and confession of secrets. As the series progresses, her role shifts from a distant conductor to a more active participant in the emotional connections she helped create, especially after her own Kiznaiver scar is revealed.

Her most significant relationship is with Katsuhira Agata. They share a deep past as childhood friends and test subjects in the original experiment, but Katsuhira has no memory of her. Noriko feels an intense connection to him and is visibly affected by his growing awareness of her. Their bond is the emotional core of her development, culminating in Katsuhira declaring his love for her and their reconciliation. She also has a strained relationship with Chidori Takashiro, who views Noriko as a rival for Katsuhira’s affection. Noriko interacts respectfully but distantly with Mutsumi Urushibara, the researcher who cares for her and oversees the project.

Over the course of the series, Noriko undergoes subtle but significant emotional growth. Initially, she appears incapable of empathy and speaks only in terms of data and results. As she spends more time with the Kiznaivers, she begins to question her own detachment and eventually stops taking her medication in an attempt to feel again. This leads to a breakdown in her composure, but also allows her to genuinely connect with the group. In the end, she launches a drastic plan to forcibly link everyone in the world, but Katsuhira’s intervention helps her realize the importance of voluntary connection and emotional honesty, leading her to accept a more human and vulnerable state.

Noriko’s notable ability is her control over the Kizna System, which allows her to initiate, maintain, and manipulate the pain-sharing links between individuals. She can use this system to transmit physical and emotional sensations among connected Kiznaivers, as well as to absorb pain herself due to her permanent link with Katsuhira. However, she does not possess conventional combat skills or physical prowess; her power is primarily psychological and systemic, relying on manipulation, precise instructions, and the authority she holds over the experiment.