Mackie Stingray is a central supporting character in the Knight Sabers' operations, serving as the team's mechanic, technical advisor, and younger brother figure to the group's leader, Sylia Stingray. His true nature is one of the series' most significant revelations, as he is not an ordinary human. Mackie is actually a prototype artificial life form, a unique creation designed by Sylia's father, the brilliant but morally ambiguous scientist Dr. Katsuhito Stingray. In his quest to advance artificial intelligence and android technology, Dr. Stingray created Mackie as a male doppelganger, using Sylia's own brain patterns as a model. However, Mackie was deemed a failure because he was considered too human, possessing emotions and a sense of self that were not desired for a mass-produced Boomer unit. Despite this, he was not discarded and was instead raised alongside Sylia as her younger brother, a role she fully accepts even after learning the truth about his origins.

Mackie is depicted as a naive, inquisitive, and somewhat awkward young man who possesses a deep and genuine love for machinery and computers. He is a prodigy when it comes to technology, a trait that proves invaluable to the Knight Sabers. Largely sheltered from the harsher realities of the vigilante lifestyle by his overprotective sister, Mackie nonetheless demonstrates a strong desire to help and be involved. He is polite and well-meaning, often providing a calm and earnest presence in contrast to the more hot-headed members of the team. One of the more poignant aspects of his character is his inability to age; because he is an artificial being, he is forever trapped in a mid-teen form, unable to experience the normal passage of time that defines the human condition.

Mackie's primary role within the narrative is to serve as the Knight Sabers' technical support. He works alongside Nigel Kirkland, a sullen but brilliant mechanic and former Genom engineer, at a rundown motorcycle shop. Together, they are responsible for building, maintaining, and repairing the advanced hardsuits that the Knight Sabers use in combat. Mackie also takes care of Priss Asagiri's motorcycle, a task made difficult by her aggressive driving style. Beyond maintenance, he is capable of piloting his own yellow utility powered armor, and he often drives a large transport truck that deploys the Knight Sabers and their equipment to mission sites. His skills with computers also make him an effective hacker and analyst, often assisting Nene Romanova with intelligence gathering.

His key relationships are central to his character development. His bond with Sylia is the most important, as she is both his sister and protector. She initially tries to keep him out of the dangerous world of the Knight Sabers, but she eventually accepts his role on the team. Mackie also develops a close working relationship with Nigel, who becomes a mentor figure in the mechanical arts. Over the course of the series, he shows clear romantic feelings for Nene Romanova, the team's hacker. This relationship helps him explore his own humanity, as he navigates the emotions of a typical teenage crush despite his artificial origins. His character arc revolves around the discovery of his own past. As the series progresses, the circumstances of his creation and his direct connection to the powerful and enigmatic Boomer known as Galatea are revealed, forcing him to confront the question of what it truly means to be human. Through this journey, Mackie comes to terms with his identity not as a failed experiment, but as an individual with his own will and a valued member of his found family.