Eosinophil is a white blood cell specializing in defending against parasitic infections and regulating allergic responses. She primarily targets large pathogens like parasitic worms, contrasting with her limited effectiveness against common bacteria and viruses.

Physically, she appears as a youthful female cell with blonde hair in pigtails, gold eyes, and a distinctive pink uniform. This pink uniform corresponds to the eosin staining used in laboratory identification, while her pigtails visually echo the bilobed nucleus characteristic of eosinophils under microscopy. She carries a sasumata, a non-lethal pole weapon symbolizing her role in containing parasites without excessive tissue damage.

Her personality exhibits a reserved and serious demeanor during duties, often appearing emotionally restrained. She displays noticeable shyness and flusters easily when receiving praise or attention. This timidity is compounded by occasional mockery from other immune cells regarding her perceived combat weaknesses against common pathogens.

A defining moment occurs during an infection by Anisakis larvae. Initially underestimated after struggling against Vibrio bacteria, she later single-handedly eliminates the Anisakis parasite when other cells retreat, earning recognition and apologies. Her proficiency with the sasumata proves essential.

Biologically, her combat involves releasing cytotoxic proteins—major basic protein, eosinophil cationic protein, and eosinophil-derived neurotoxin—from intracellular granules to neutralize parasites. These substances can contribute to tissue damage during allergic reactions. Her activities are regulated by cytokines, particularly interleukin-5 (IL-5), which promotes production and activation.

Eosinophils constitute only 1-3% of circulating white blood cells, explaining her infrequent appearances. They primarily reside in tissues like the gastrointestinal tract and respiratory system, positioning them to respond rapidly to invasive parasites.