Kuku is a child raised at the Takahara Academy, a secluded facility on Izukunoe Island that houses multiple generations of specially engineered children. As part of the third generation, Kuku is approximately ten years old and is physically small, with shoulder-length black hair and black eyes. The character is officially described as intersex and belongs to the Hiruko species, a term tied to the inhuman nature of the facility's children.

Kuku has an energetic and adventurous personality that often leads to impulsive actions. Curiosity drives much of her behavior, and she rarely hesitates to break rules or explore off-limits areas. This restlessness makes her both endearing and unpredictable among her peers, as she can easily drag others into her escapades without fully considering the risks.

Her primary motivation appears to be a desire to uncover what the adults are hiding. This is most evident when she secretly leads Tokio, a second-generation child, to a hidden chamber containing experimental, featureless babies. Kuku’s willingness to cross boundaries is not rooted in malice but in a genuine need to share what she finds and to push back against the ignorance the facility imposes on its residents. Beyond simple mischief, her actions suggest a loyalty to her friends and a quiet rejection of the controlled environment she has grown up in.

In the larger story, Kuku functions as a major supporting character whose arc parallels the central mystery of the academy. Her most significant role unfolds after her death, when she transforms into a fish-like man-eater—one of the monstrous creatures that plague the outside world. The first direct encounter between the series’ traveling protagonists, Maru and Kiruko, and a man-eater on a boat involves Kuku’s posthumous form, making her a tragic bridge between the facility’s hidden horrors and the wasteland beyond. Her presence in the narrative thus illuminates the true nature of the academy’s children: they are not merely students but the origins of the world’s monstrous threats.

Kuku’s key relationships center on Tokio and, to a lesser extent, the other third-generation children. With Tokio she shares a bond built on exploration and mutual curiosity. She is protective of Tokio, guiding her through the facility’s secrets and even using her own agility to help them evade detection. Among her generation, she is a spirited presence, though her impulsiveness can place her at odds with the more cautious members of the group. Her connection to Tarao, another third-generation child who warns Tokio to escape the dangerous facility, suggests she belongs to a network of children who are beginning to doubt their caretakers.

As a character, Kuku does not undergo an extended internal development, but her trajectory is one of revelation and loss. Her actions trigger Tokio’s deeper awareness of the facility’s inhuman experiments, and her death and subsequent transformation make the cost of those experiments viscerally real. In this way, her fate is a pivot for the audience’s understanding: the cheerful, agile child becomes a mindless, water-dwelling monster, underlining the cruelty of the academy’s design.

Regarding notable abilities, Kuku exhibits superior agility and physical coordination even before her transformation. She moves quickly and silently enough to bypass security measures, and she is described as a very agile individual within the facility. As a Hiruko child, she possesses latent inhuman traits that manifest most drastically after death, when she metamorphoses into a large fish-like man-eater with suction-cup hands and a body that requires water to survive. This posthumous form retains no trace of her original personality, but it confirms that the academy’s children are the source of the monstrous anomalies scattered across the broken world.