Eiko Magami, most often called A-ko, is the central character of her series and a sixteen-year-old transfer student at Graviton High School in the city of Graviton. She is an athletic girl with thick red hair that falls past her shoulders and is often seen wearing a distinctive white sailor-style uniform from her previous school, complete with a red scarf, a blue pleated skirt, and shiny black Mary Jane shoes. A-ko is typically cheerful, friendly, and energetic, but she also possesses a famously fiery temper that can make her quite dangerous when provoked. Despite her extraordinary abilities, she yearns for a perfectly normal life, daydreaming about finding a cute boyfriend and enjoying a romantic summer free from the disruptions that frequently plague her. Her daily life is further complicated by a few common flaws, including a tendency to chronically oversleep, a struggle with punctuality, and a very large appetite. A-ko’s primary role in her story is that of a reluctant heroine who is repeatedly pulled into chaotic conflicts, whether by a jealous school rival or a full-blown alien invasion, all while simply trying to get through her school day.

A-ko’s most defining relationship is with her best friend, C-ko Kotobuki, whom she has known since kindergarten. Their bond is one of deep loyalty, with A-ko acting as a protective older sister to the immature and often exasperating C-ko, whose safety is frequently the catalyst for A-ko’s most violent outbursts and heroic actions. The central conflict of the series is driven by her intense rivalry with her classmate, the wealthy and brilliant B-ko Daitokuji. B-ko is obsessed with winning C-ko’s friendship and sees A-ko as an obstacle, leading to daily battles where A-ko uses her raw power to destroy B-ko’s advanced mecha. In later sequels, this rivalry expands to include competing for the affection of a young man named Kei, further escalating their clashes. However, when faced with a common threat, such as aliens attempting to kidnap C-ko, these two rivals will set aside their differences to work together. An alternate continuity portrays an older, nineteen-year-old A-ko as a bounty hunter who has become a vitriolic partner and roommate to B-ko, demonstrating how their dynamic can shift into a grudging friendship. Over the course of the series, A-ko shows some development in her personal desires. In Project A-ko 3: Cinderella Rhapsody, she becomes preoccupied with appearing more feminine, taking a part-time job to buy new clothes in a focused attempt to attract a boyfriend, which strains her friendship with C-ko before she ultimately reconciles and abandons the pursuit. In Project A-ko 4: Final, she follows C-ko into space to prevent her from leaving Earth.

A-ko possesses immense superhuman abilities, making her a formidable force. She has incredible superhuman strength, allowing her to lift and throw tanks and mecha with ease, tear through titanium, and create fissures in the ground with a flick of her wrist. She is also a lightning-fast speedster capable of running at hypersonic speeds, leaving a trail of destruction like shattered pavement in her wake as she races to school. Her body is nigh-invulnerable, granting her the durability to withstand missile barrages, crash through brick walls, and survive being crushed by giant robots. To manage her uncontrollable power, A-ko wears special black wristbands that act as power limiters. When these are removed, her strength becomes catastrophically destructive, and even a simple gesture can level buildings. Her white socks also feature small decorative weights called bon bons made of uranium that help keep her power in equilibrium, while her Mary Jane shoes are designed to enhance her speed and jumping ability. Beyond raw power, her iconic sailor uniform ultimately provides her with the ability to breathe and survive in the vacuum of outer space.