Nayuta Hida is a genius scientist and the inventor of the Heart Hybrid Gear technology, which is the only effective weapon against invaders from another universe. She is the biological mother of Kizuna Hida and Reiri Hida, and was formerly the guardian of Aine Chidorigafuchi. Described as a charming woman with long black hair and a youthful appearance that makes her look nearly the same age as her daughter, she was once widely renowned for her work before she disappeared under unknown circumstances.

Her personality is that of a quintessential mad scientist. She is driven by an insatiable curiosity and a desire to study anything that captures her interest, regardless of the ethical or humane cost. She has been described as amoral and willing to use inhumane methods in her research. This nature put her at odds with both of her children, whom she abandoned when she no longer had a use for them. She admitted that the only reason she had children was to see if it would change anything about herself. Her motives are often ambiguous, causing distrust even among allies such as the Vatlantis Empire. Her ultimate goal is to become a goddess so that she can recreate the worlds as she sees fit, which positions her as one of the main antagonists of the series.

In the story, Nayuta initially appears in Kizuna’s dreams, where she tells him she no longer needs him for her future experiments. She later reappears in person, revealing that she has been working in the Alternate Universe alongside the Vatlantis Empire. Her role is that of a hidden mastermind behind many of the conflicts faced by the protagonists. She is the creator of the Heart Hybrid Gear, a technology that extracts life force from its users, and she is responsible for the many battles the characters endure. Her actions force the main characters to confront the true nature of their powers and the cost of their fight.

Her key relationships are with her children, Kizuna and Reiri. She abandoned both of them, and they view her with a mixture of resentment, caution, and fear. She also has a distant, transactional relationship with Aine, whom she once guarded. Her alliance with the Vatlantis Empire is one of convenience, as she uses their resources to further her own research. She is guarded by a bodyguard named Valdy, who wears a magic armor called Rael that can manipulate space.

Nayuta undergoes a notable physical transformation later in the story: after transforming into a machine god, her body regresses to that of a preteen, her eyes turn golden, and she begins wearing a strange kimono. This change reflects her evolution from a human scientist into something beyond human, aligned with her goal of godhood.

Her most notable ability is her genius-level intellect, which she used to invent the Heart Hybrid Gear and its core technology. She also possesses extensive knowledge of magic and science, gained from her work with the Vatlantis Empire. After her transformation, she gains the abilities of a machine god, which include enhanced power and the capacity to manipulate reality on a world-altering scale. Her bodyguard Valdy’s spatial manipulation abilities are also a direct result of her research. Overall, Nayuta Hida is the hidden architect of the series’ central technology and the primary source of the overarching conflict, driven by a cold, relentless pursuit of ultimate power and knowledge.