Komoe Harumachi is an 11-year-old sixth-grade student at Zashono Academy with long pink hair styled in prominent arching pigtails and matching pink eyes. She stands 4 feet 9 inches (145 cm) tall and weighs 110 pounds (50 kg), with physical proportions measuring a 43-inch (111 cm) L-cup bust, an 18-inch (46 cm) waist, and 25-inch (65 cm) hips. Her substantial bust size results from macromastia, a medical condition causing extreme breast growth that contributes to physical challenges. Standard bras prove inadequate, requiring her to use tight-fitting sports bras for support.

Komoe exhibits a kind and considerate personality, eagerly assisting the Eiken Club to feel useful and needed. She finds satisfaction in cooking, cleaning, and contributing to club tasks. Her large bust disrupts her balance, leading to frequent clumsiness. This often results in accidental falls, exposures, or incidents where she lands on and squashes fellow club member Densuke Mifune. She dislikes fainting, feeling burdensome, or being perceived as useless due to these mishaps.

She shares a sibling-like relationship with Densuke, treating him as a brother figure. Their interactions frequently involve accidental groping or awkward situations, such as Densuke mistaking her breasts for objects like snowballs. Komoe enjoys spending time with him, alongside activities like using the school pool or jumping rope.

Komoe's mother shares a similar large bust size, suggesting a hereditary component to her condition. She relies on prescription glasses for clear vision, owning two pairs and using the stronger pair to distinguish faces. Without glasses, her vision is significantly impaired.