U-1196 operates as a neutrophil in a human body ravaged by smoking, excessive alcohol, and chronic stress. These conditions create a perilous environment demanding unceasing vigilance against pathogens and internal damage. She combats bacterial invasions with specialized katana-like swords, mirroring neutrophils' pathogen-engulfing function. Maintaining a strictly professional, serious, and mission-driven demeanor, she never complains despite the body's decay. Her appearance includes an open uniform top without an undershirt—a style common among local neutrophils—and a notably curvy physique. The numbers "96" in her name subtly evoke the Japanese word "kuro" (black), aligning with the narrative's bleak tone.

Originally partnered with neutrophils U-1212 and U-8787, U-1196 repeatedly rescues rookie red blood cell AA2153 during patrols, forging a bond of professional trust rare in an environment where other cells distrust white blood cells. After her partners die during overwhelming infections, she shoulders duties alone. Her dedication verges on self-sacrifice, exemplified when leading a costly, failed assault on a urate crystal during a gout crisis—a defeat that briefly shatters her confidence. AA2153’s emotional condemnation of the body’s neglectful host renews her resolve.

Severely wounded fighting periodontitis bacteria, U-1196’s recovery allows cynical junior neutrophil J-1178 to witness her unwavering commitment firsthand. J-1178 adopts her as a mentor, dubbing her "big sister," and they form the neutrophils' strongest partnership, crucial in later crises. After the first body’s terminal decline, U-1196 and AA2153 transfer via transfusion to a second unhealthy body. There, her experience and calm authority deepen J-1178’s respect and reliance. She continues her protective role with stoic dedication against relentless threats in this new hostile environment.