Mast Cell possesses a serene appearance with brown eyes and shoulder-length black hair. She wears a yellow t-shirt under a white lab coat, an olive knee-length skirt, black or brown tights patterned with grey lines, and white shoes.

Her demeanor contrasts sharply with her calm exterior; she reacts fiercely when displeased or when her work is undervalued. Insecurity plagues her, especially the fear that her role's overactivity causes harm. This insecurity peaks when others mistakenly call her "Fat Cell"—a term she despises due to its inaccuracy regarding her function. For instance, in Episode 5, B Cell blames her for collateral damage and uses this misnomer, provoking her to retaliate with "Bonehead" before their clash escalates physically.

Functionally, she is a granulocyte and white blood cell. Her core duty involves regulating the release of chemical mediators like histamines and leukotrienes, activated by excessive Immunoglobulin E antibodies from B Cells. This process is vital for countering allergens.

In the pilot chapter "The Story of Cells," she emerges during a dispute involving Memory Cell, Antibody-Producing Cell, Killer T Cell, and Helper T Cell. The confrontation triggers an excessive histamine release, highlighting her tendency to overreact under pressure.