Saku Momoi, nicknamed "Loli," is a high school student distinguished by her remarkably youthful appearance, standing 142 cm tall and weighing 35 kg. Her petite frame and childlike innocence consistently lead others to mistake her for an elementary school student, making her a recurring target for misunderstandings in adult situations she fails to grasp.

She resides with her grandmother due to her parents working overseas in Germany, a defined aspect of her background.

Her unawareness of mature topics like human relationships or basic anatomy repeatedly lands her in uncomfortable predicaments. She attracts unwanted predatory attention from Lily Someya, who exploits Momoi's perceived innocence—exemplified when Lily entered "predator mode" during a bra fitting and fondled her, an act Momoi failed to recognize as inappropriate. Similarly, her attempt to reshape her eyebrows resulted in a botched appearance, eliciting suppressed laughter from peers recognizing her naivete.

Momoi harbors documented sensitivity about her physical development, particularly her height and chest size. This insecurity is frequently heightened by Nozomu "Baka" Tanaka, who openly ridicules her stature, provoking defensive reactions where Momoi recoils or guards herself around him. Despite this, she exhibits perceptiveness and empathy toward friends, offering insightful advice or encouragement. She channels expression through artistic ability, notably painting.

Her interactions center on navigating social challenges arising from her appearance and innocence. While her naivete brings vulnerability, she displays resilience in enduring teasing and awkward encounters.