Shiina Mayu is a junior high school student who avoids school due to struggles with human connections. Her only trusted companion was a pet ferret named Myuu, whose death occurs just before she encounters Kouhei Orihara and his friends. This loss plunges her into profound grief, manifesting in uncontrollable crying and distressed shouts of Myuu's name.

Physically, she has short brown hair with spiky bangs, blue eyes, a slim build, and a pale complexion. She typically wears a school uniform. Her personality blends immaturity and kindness, with a love for nature and food—especially teriyaki burgers. She fixates on long, slender objects reminiscent of ferrets, sparking fascinations with Rumi Nanase's hair and plush snakes.

Kouhei intervenes to foster her maturity, integrating her into his high school class despite her younger age to encourage social growth and diminish her reliance on Myuu's memory. Initially prone to tantrums and emotional outbursts, her exposure to Kouhei's social circle gradually nurtures human friendships, marking her progression toward emotional resilience. Her background includes a stepmother, later identified as Kaho Shiina in supplementary materials.

Mayu regularly visits Myuu's grave, reflecting lingering attachment, though narrative sources indicate her journey involves learning to navigate relationships beyond the ferret bond.