Pierrot recurs as a human character throughout the "Once Upon a Time..." franchise. In the frame narrative of "Seimei no Kagaku: Micro Patrol," he is a young boy encountering everyday health and biology scenarios—like injuries, illnesses, or routine activities—that pivot into educational segments about bodily functions.

Within the microscopic human body setting, Pierrot transforms into an anthropomorphized lymphocyte B named "Captain Peter." He commands a patrol craft featuring hydrojet propulsors and an underbelly bomb bay for antibody deployment. His duties involve monitoring the body, detecting pathogens, and directing immune responses with fellow defense cells. His design merges human traits with immunological context, including a light blue uniform and lymphocyte B squadron shoulder pads.

Pierrot’s dual role connects the human-scale stories to microscopic adventures: his real-world injuries or illnesses trigger explorations of biological systems, where Captain Peter combats bacterial and viral threats. This mirrors his earlier portrayal in "Once Upon a Time... Space" as the son of political figures Colonel Pierre and President Pierrette, maintaining consistent identity across the franchise.