Yayoi Sanzenin is the school nurse at Koyomi Academy and the thirteenth-generation miko of the Tonami-style Sanzenin tradition. Her parents serve as kannushi at the family temple, where she assists outside school hours. During school, she wears a long white nurse coat and skirt, switching to her family's traditional Japanese priest attire at home.

Born into this lineage, Yayoi possesses heightened spiritual awareness and extensive kendo training, enabling her to sense supernatural disturbances and react with combat readiness. Her typically calm, nurturing nurse demeanor shifts to protective aggression when perceiving threats to student Chitose Hitotose. This often manifests as her katana flashing to his neck to compel compliance, defining her disciplinary approach.

Within Chitose's household, she participates in collective motherly duties alongside four other resident teachers, sharing domestic responsibilities. Her primary narrative function is safeguarding Chitose's wellbeing through spiritual and physical means. Male students frequently feign illness to receive her care, leading to her "Hospital Hottie" designation, though this remains incidental to her core relationships.

Throughout the series, including "Happy Lesson The Final" OVA, her character remains consistent. She participates in group efforts to save the school from demolition, aligning with her guardian role. A notable deviation occurs in episode 4 during a competitive eating contest against Satsuki Gokajo, showcasing her capacity for rapid consumption of large food quantities, though this trait does not recur.

Her interactions reinforce her archetype: collaborating with the teacher group on shared duties toward Chitose, occasionally clashing with colleagues over methods, and uniformly directing protective actions toward Chitose.