Mary Steinberg is a young augmentation engineer who performs maintenance and repairs on individuals known as Extended, people who have undergone cybernetic body modifications. She operates out of the Kyusei Pit, a mafia-controlled immigrant district that remains outside the influence of the Berühren Corporation, making her one of the few independent mechanics in the city. Her work often involves emergency cases, and she is known for her high level of skill and resourcefulness.

Born an orphan, Mary and her older brother Victor were taken in by their uncle. Victor chose to follow their uncle’s path as an augmentation engineer, but his passion for improving the technology led him to the battlefield, where he developed a new system called the Parallel Assist Brain. A tragic incident occurred when Victor’s own body was taken over by the auxiliary brain, warping his original personality and causing him to vanish. Mary became a mechanic specifically in the hope of one day finding her brother and bringing him back. This personal quest is the central driving force behind her actions throughout the story.

Mary has a pragmatic and direct personality, often speaking her mind without overthinking. She can be flirtatious and teases the protagonist Juzo Inui, with whom she shares a childhood friendship. Despite her sometimes sharp or playful manner, she is deeply loyal and kind. When Juzo’s life is in danger, she rushes to help him, even while hiding her own identity in some situations. She also holds a strong philosophical belief that even though augmentation can change a person’s body, human beings have the inner strength to change for the better and should never let technology dictate their souls. This conviction puts her in direct opposition to her brother’s later nihilistic aims.

Mary’s role in the story is that of a crucial ally and technical support for Juzo and others. She provides him with medicated cigarettes when he loses access to his usual brand, repairs his damaged body after battles, and uses her connections with the local mafia to procure affordable, high-quality parts for her clients. She also helps Tetsuro Arahabaki regain mobility in his limbs through augmentation. Her knowledge and skills are considered top-notch, and she willingly performs back-alley maintenance in an area with poor security, ensuring that Extended who cannot afford corporate care still receive help.

As the series progresses, Mary undergoes notable development. She eventually installs an Extension on herself that replaces part of her torso with two missile launchers, a modification she uses in direct combat. This change not only increases her offensive capabilities but also symbolizes her willingness to put herself on the line for the people she cares about. Her search for her brother Victor becomes increasingly complicated when she discovers that the Victor she knew may no longer exist, and that the technology he created has caused widespread suffering. Despite this, she remains determined to stop him from harming others and to find the true Victor somewhere inside the altered being.

Key relationships include her deep bond with Juzo Inui, who trusts her implicitly and relies on her expertise. She also forms a protective connection with Tetsuro Arahabaki, the young boy at the center of many conflicts. Her most important relationship, however, is with her brother Victor. The tragedy of their shared past and her desperate hope to save him from his self-destruction defines much of her emotional arc. She is also portrayed as a mood maker within the group, providing moments of levity and warmth amid the grim cyberpunk setting. Mary Steinberg is a capable, determined, and compassionate engineer who uses her talents to support her allies while pursuing a personal mission that tests her beliefs and her strength.