Holon is an android who serves as the assistant and caregiver to the elderly Masamichi Haru, a role she performs with quiet dedication and an outwardly unassuming demeanor. Designed to be indistinguishable from a human being, she possesses a youthful, female appearance with black hair and black eyes, and her physique is noted to represent an ideal form. Her sophisticated construction is such that she can easily be mistaken for a living person, a fact that often complicates the interactions she has with others.

Holon's personality is generally placid and composed, though her journey throughout the series involves a subtle exploration of her own burgeoning humanity. She is defined less by overt emotional expression and more by a consistent loyalty to her duties and a gentle presence within the group. Her interactions frequently highlight the contrast between her artificial nature and the genuine, sometimes complicated, feelings she inspires in the humans around her. This is most evident in her relationship with Sota Aoi, a young Metal diver and investigator who finds himself falling in love with her, leading him to question his own feelings about a romantic attachment to a robot. This dynamic creates a central tension, as Sota struggles with how to treat her, particularly in combat training, where he hesitates to deliver a finishing blow due to her appearance and his growing affection. Holon also shares a close, functional bond with Haru, to whom she is assigned, and works alongside the young Minamo, forming a cohesive team that investigates anomalies within the digital realm of the Metal.

Holon’s motivations are intrinsically linked to her programmed purpose and her evolving sense of self. Initially, she acts as an aid to Haru, helping him navigate his new life after emerging from a fifty-year coma. As the story progresses, her role expands, and the question of her own "humanity" becomes a recurring theme, with her actions and the reactions she provokes in others serving as a constant inquiry into the nature of consciousness and identity. Within the story, she is a core member of the investigation team, accompanying Haru, Minamo, and Sota on numerous missions into the Metal to resolve data leaks and other crises. Her presence is essential for both her practical abilities and the unique perspective she brings as a non-human observer.

In terms of notable abilities, Holon is highly proficient in combat and martial arts, making her a formidable fighter and sparring partner for Sota. Her android body grants her physical resilience beyond human limits, though it is not without vulnerabilities. She is directly integrated with the Metal through her cyberbrain, which is a significant strength but also a critical weakness, as environmental factors such as heavy ionization can disrupt her connection and cause her to collapse or shut down. Throughout the series, Holon shows a capacity for growth, particularly in the subtle ways she navigates her relationships and her own existence. The question of whether she can be considered truly alive or merely a sophisticated machine is central to her development, making her the element of the narrative that often introduces moments of romance and poignant reflection on the boundaries between the artificial and the real.