Mahoro is the central character of her story, a powerful combat android created by the organization known as Vesper to fight against an alien force called Saint. Her designation is Code V1046-R MAHORO, and she was constructed using alien technology that granted her not only immense physical capabilities but also a genuine soul and a pure heart. After a long and successful career as a soldier, Mahoro is informed that her operational lifespan is limited, and she has only 398 days remaining before she will cease to function. Given the freedom to choose how to spend her final days, she makes the decision to leave her life as a warrior behind and become a maid for Suguru Misato, a young boy whose father was her former commanding officer. This choice is laden with guilt and a sense of duty, as Mahoro was forced to kill Suguru's father when an alien took him hostage, acting on his own command to do so.

Mahoro possesses a sweet-natured, well-mannered, and kind-hearted personality, quickly becoming a beloved and friendly presence in her new community. She is an exceptional cook, able to create nutritious and delicious meals from even the most humble ingredients, and she maintains the Misato household with dedicated care. Her cheerful and approachable demeanor makes her a popular figure, even at Suguru's school, where word of his live-in maid soon spreads. Despite her gentle nature, Mahoro has a strict and unwavering intolerance for anything she considers perverted, a trait instilled in her by Suguru's father, who taught her that "dirty thoughts are bad". This manifests most comically in her relentless confiscation of Suguru's hidden pornographic magazines, which she is somehow always able to find. She is famously known for her catchphrase expressing this sentiment. Her protective instincts extend to guarding Suguru from the amorous advances of his teacher, with whom she shares a mutual dislike.

Mahoro's primary motivation is to devote her remaining time to making Suguru happy and providing him with a stable, caring home life, an act that serves as her penance for the death of his father. This desire for a peaceful existence is constantly at odds with her original programming as a weapon. She is a formidable warrior, still retaining access to all her combat abilities and equipment. She can summon a flying unit called the Silpheed and her support mecha, Slash, who resembles a large, intelligent black cat, from the badge she wears on her uniform. She is also armed with a powerful pistol, and can call upon a fully equipped and armed BMW sports car and motorcycle. Her most devastating weapon is an energy whip linked directly to her core; using it, however, drastically shortens her remaining lifespan.

In the story, Mahoro serves as both the caretaker and protector of Suguru, as well as a living reminder of the hidden war that exists alongside everyday life. Her role forces her to constantly balance her duties as a maid with the need to fend off threats from her past, including other combatants who seek to challenge her. Her most significant relationship is with Suguru, which is complex and layered; she acts as a doting mother, a devoted servant, and a shy, blushing girlfriend all at once. She also has a connection to a presence known as Matthew, whose memories are linked to her and who occasionally provides her with advice. As her limited time ticks away, Mahoro undergoes significant development, struggling more deeply with her dual identity as a warrior and a maiden. She begins to fully explore what it means to be human, confronting her own feelings, her fears about the end of her life, and the profound desire for a simple, meaningful connection that transcends her nature as an android.