Mahiru Hyuga is a teenage girl who comes from the Hyuga family, one of the two primary and powerful families in the secluded Karakami Village. She is a Seki, a person with the ability to control a kami, or a kakashi, a massive wooden doll-like entity. Her designated kakashi is named Magatsuhi. Physically, she is a beautiful young girl with distinctive curly black or green hair, often dressed in attire that carries a slightly gothic vibe, and she is typically seen wearing her school uniform.

Mahiru possesses a personality that stands in stark contrast to her cute and elegant appearance. She is characterized by an extremely strong-willed and violent temperament, often described as having the nature of a junkyard dog. Having been raised in a highly classist and strict family environment that employed forceful methods of upbringing, she developed a very rough and arrogant demeanor from a young age. She is known for her rude speech and actions, showing little respect for authority or tradition, even treating her own cousin, Koshiro Hyuga, the heir to the family, with discourtesy. Her temper is exceptionally short, leading her to lash out and go on rampages when provoked, and she has no qualms about kidnapping people to achieve her ends.

Despite this fierce and ruthless exterior, Mahiru has a singular and notable exception to her contempt for others: Kyohei Kuga. Her motivations are heavily centered on him. She holds an obsessive infatuation and admiration for Kyohei, which stems from a traumatic event in their shared childhood. On that day, Kyohei saved her life and the life of Aki Kuga by awakening and unleashing the true power of his own kakashi, Kukuri. From that moment on, Mahiru has regarded Kyohei as the strongest Seki and a brave hero. Her primary goal is to topple the ancient and rigid ruling system of Karakami Village, not for altruistic reasons, but so that she can be free to be with Kyohei. She actively seeks to break down the old customs and class systems that keep their families separate.

In the story, Mahiru is initially a mysterious presence who appears in the city away from her village. She enters the narrative decisively by kidnapping the protagonist, Hibino Shiba, to interrogate her about her relationship with Kyohei. This action brings her into direct conflict with the other characters. She shows a particular cruelty towards Utao Kuga, callously crushing a box of cookies Utao had made, which provokes a fight. Despite facing multiple opponents, she proves to be a formidable combatant, easily shrugging off attacks until a combined effort is required to stop her. Later in the series, after losing her own kakashi, Magatsuhi, in battle, she becomes a homeless girl stealing from convenience stores, stripped of the high status she once had. However, her pride and fierce personality remain unchanged.

Mahiru’s key relationships define her actions. Her most significant connection is with Kyohei Kuga, whom she follows and praises with a deredere, or love-struck, attitude that contrasts starkly with her usual behavior. She treats Koshiro Hyuga, her cousin, with rudeness despite his position. She has a working relationship with the politician Takeshi Hirashiro, who sees the power of the kakashi as a tool for his own political ambitions, though her interactions are often tense due to her uncontrollable nature. She also shares a history with Aki Kuga, as they were both saved by Kyohei as children, and they become allies of circumstance, both being outcasts from the village system at one point.

Throughout her development, Mahiru experiences a significant fall from grace. She loses her powerful kakashi, Magatsuhi, which is destroyed in a fight, forcing her to abandon her family home and live on the streets. This period of homelessness humbles her circumstances but does not soften her spirit. She later seizes an opportunity to take control of a different and powerful kakashi named Takemikazuchi, showing her resourcefulness and continued ambition. Her actions reveal that beneath her selfish and arrogant exterior, she is capable of a certain degree of loyalty and care, as she eventually goes to rescue someone despite initially refusing.

As a Seki, Mahiru possesses notable abilities. She is considered the most powerful Seki of the Hyuga family and excels at controlling her doll, Magatsuhi. Her confidence in her own abilities is so great that she considers all other Seki, with the sole exception of Kyohei, to be inferior to herself. Magatsuhi itself has a special ability that can paralyze or stop the movements of other kakashi, making her a particularly dangerous opponent in battle. She also demonstrates high physical durability, being able to shrug off attacks that would fell other characters.