Koyomi is a fully human orphan who appears as a recurring figure throughout the Battle Angel Alita manga and its sequel, Last Order. She was adopted as a baby by the proprietor of Bar Kansas in the Scrapyard, and her family background remains unknown. Her birth in ES 576 is used as a chronological anchor for the series, with many events dated relative to her age. She has no cybernetic enhancements, which is unusual in the harsh, machine reliant world of the Scrapyard.

In her earliest appearances, Koyomi is shown as a baby, toddler, and young child. As a baby, she is kidnapped by the criminal Makaku to lure Alita into battle, but is saved by her adoptive father's dog, Duke Fang. As a toddler, she forms an affectionate bond with the hunter warrior Murdock and his dog Fury, demonstrating an empathetic nature that softens those around her. By the time she is thirteen, Koyomi has grown restless at home. She runs away with Fury and joins a caravan, hoping to meet the outlaw radio broadcaster Kaos, whom she idolizes. During this journey she defends the caravan by outwitting bandits in a gambling game, showing a sharp mind and courage. She later falls into an underground ruin with Alita, an event that brings her face to face with Kaos and his followers. When Alita, then acting as a TUNED agent for the floating city of Tiphares, is about to execute the rebel leader Den, Koyomi leaps between them, placing herself in harm's way to protect Den. She openly joins the Barjack rebellion, convinced by Den's critique of Tiphares. Alita withdraws, and Koyomi becomes a Barjack soldier.

As a member of the Barjack, Koyomi witnesses major campaigns, including the firing of the railway gun Heng at Tiphares and its destruction by the Abaddon. She saves the information broker Vector from execution, and after the Barjack disbands, she nearly follows Den on his suicide run into the Scrapyard, but is thrown from his vehicle when Den is confronted telepathically by Kaos. Her foster father catches her, breaking both his legs, and she survives. A year later, Koyomi compiles the photographs of the fallen journalist B.B. Buick into a best selling book titled Barjack War Chronicle, ensuring the rebellion's story is preserved. She then becomes a freelance photographer for the Screwhead Times and plans books on the Tower of Tiphares and on Alita herself. In Last Order, set when Koyomi is fourteen, she boards with her father in an antiques shop, encounters a broken Kaos, and helps him by returning his sword and blueprints. She assists in transporting Kaos and witnesses his transformation when he receives Alita's paper airplane from Tiphares.

Throughout her life, Koyomi is defined by determination, adaptability, and fierce loyalty. She runs away from security to chase her ideals, stands up to seasoned fighters, and repeatedly risks her life for those she believes in. She is not a combatant in the traditional sense; her strengths lie in her quick wits, empathy, and later her documentary eye. Her relationship with Alita is central. Alita recognizes her even after an eleven year gap, and Koyomi's actions directly challenge Alita at critical moments. She also shares a deep protective bond with her foster father and with the dog Fury. Her encounter with Kaos shapes her understanding of the world's power structures, and her time with the Barjack cements her resolve to tell hard truths through photography. Koyomi evolves from a wide eyed child into a resilient chronicler of war, using her camera to give voice to the struggles of the Scrapyard.