Red Blood Cell AE3803 transports oxygen, carbon dioxide, and nutrients throughout the body. She possesses bright red hair, amber-brown eyes, and wears a reversible jacket that changes color with her cargo: bright when carrying oxygen, dark when transporting carbon dioxide. Her hat, shaped like a red blood cell, displays her identification code. The "AE" in her code references a hemoglobin E trait, a mild genetic anemia potentially contributing to her directional challenges and inefficiency in gas transport.

Teased as an erythroblast in the bone marrow for frequently getting lost, she once wandered into danger during training and was attacked by a Pseudomonas aeruginosa bacterium. A young myelocyte, later implied to be Neutrophil U-1146, intervened despite being outmatched, enabling both to survive until reinforcements arrived; they parted with a promise to reunite. After enucleation and maturation into a full erythrocyte, she began delivery duties but continued struggling with navigation, often requiring rescue by U-1146 or guidance from senior colleague AA5100.

Her personality blends clumsiness and determination. She frequently apologizes for errors and exhibits extreme reactions to threats yet remains committed to her responsibilities. During crises like hemorrhagic shock, she persists with oxygen deliveries despite near-fatal exhaustion. Over time, she evolves from a novice dependent on others into a mentor for rookie NT4201, emphasizing experiential learning and gradually improving her reliability. Her relationship with U-1146 deepens through repeated collaborations, though she initially doesn't recognize him as the myelocyte from her past.

AE3803 exhibits unique traits: her crescent-shaped ahoge sparks unconfirmed fan theories about sickle cell anemia, and she displays an instinctive attraction to danger zones, atypical for red blood cells. In the spin-off *Cells at Work! BLACK*, a separate erythrocyte named AA2153 features in a distinct narrative focused on a deteriorating body.