Mugimaru is a rabbit-like creature dispatched from Vaccine World by the Goddess Maya on a critical mission to Earth. His core function involves detecting viruses unleashed by the antagonistic Ungrar, who threatens humanity. Tasked with finding a suitable human candidate to become the Magical Nurse and wield Vaccine World's powers, Mugimaru identifies Komugi Nakahara as the only willing individual capable of accepting this role, initiating her transformation into a magical combatant.

As Komugi's designated non-human companion, Mugimaru serves as her sidekick during missions against viral threats and the adversary Magical Maid Koyori. His abilities specifically locate and identify viral infections while providing tactical support in confrontations. A distinctive verbal trait sees him append "-mugi" to sentence endings in original Japanese dialogue. Personality-wise, he exhibits playful and occasionally perverted tendencies, often generating comedic relief that aligns with the series' parody tone—a contrast to the darker themes of its predecessor, "The SoulTaker," where supported characters are reinterpreted.

His involvement continues into the sequel "Nurse Witch Komugi-chan Magikarte Z," assisting Komugi against Ungrar's schemes and Koyori's recurring threat. No significant evolution occurs in his core functions, origins, or personality across these appearances.