Natural Killer Cell is a female lymphocyte portrayed as a muscular, fit woman with short, unevenly cut black hair covering her right eye. She typically wears a black tank top, green shorts, and boots. Her standard saber transforms into a purple lightsaber-like blade when Dendritic Cell activates her.

Her personality blends aggression and sarcasm, often threatening normal cells during duties. She prefers solo work due to safety concerns for others. A competitive rivalry with Killer T Cell fuels frequent bickering and brawls over responsibilities, rooted in their complementary immunological roles. Combat effectiveness fluctuates with emotion: laughter amplifies her power, while stress drastically weakens it. She harbors embarrassment over a past affinity for gothic lolita fashion in her youth.

Functionally, she operates in the innate immune system, identifying and eliminating cancer cells, viruses, and infected cells through constant tissue patrol. Key abilities include heightened threat detection, professional saber combat, discretionary cytotoxic destruction of aberrant or healthy cells, adaptive pathogen memory, and rapid tissue transmigration. She accomplishes tasks typically requiring multiple Killer T Cells and breaches tissue walls in battle.

Despite a loner reputation, she socializes with immune cells: attending Macrophage’s tea parties alongside Eosinophil, Mast Cell, Regulatory T Cell, and Band Cell, and joining Mast Cell for hot spring trips. She discreetly protects Neutrophil and General Cell by withholding sensitive information. Her history includes a documented encounter with a male counterpart, though only the female iteration appears consistently. A timeline discrepancy notes her fighting Campylobacter in an opening theme absent from the corresponding episode.