Rea Katō, nicknamed "Rea-Pi," is a small-statured and hyperactive fifth-grade student, one of Tsubomi Tachibana's close friends alongside Yae Yamabuki. Her pronounced curiosity frequently fixates on puberty and sexuality, subjects she labels "perverted things," driving behaviors like openly inquiring about male anatomy or expressing interest in concepts such as erections.

She shares a long childhood friendship with Futoshi Kuki, who persistently teases her about her curvier physique and weight compared to peers. This teasing fuels her desire to become slimmer and more conventionally attractive. Despite Futoshi's public remarks about her appearance, their dynamic involves underlying mutual affection, with Futoshi harboring genuine feelings for her.

Within the group, Rea vocally explores mature topics, actively discussing bras, physical development, and classmates' romantic activities, including conversations about purchasing bras. She navigates body image insecurities intensified by Futoshi's comments while maintaining her outgoing, inquisitive nature, consistently engaging with sensitive subjects despite occasional social awkwardness.

Her role provides comic relief through exaggerated reactions and unabashed curiosity, while also embodying the realistic anxieties of children entering early adolescence. Her character remains consistent across the OVA adaptation and original manga, centered on energetic inquisitiveness and navigating personal body image concerns within her social circle.