Kasumi Nomura, a studious middle schooler at an all-girls school, maintains an intelligent demeanor often seen reading books in class. Her serious exterior masks significant difficulty with English; despite dedicated efforts, she performs poorly in the subject. This drives her to seek help from Olivia, mistakenly believing Olivia is fluent. Kasumi developed a lasting aversion to games due to childhood experiences where her older sister consistently defeated her, forcing her to do chores as punishment. This history fuels her initial reluctance to participate in games with others. Nevertheless, classmates Hanako Honda and Olivia frequently draw her into activities, forming the core trio of the Pastimers Club. Within the group, Kasumi often acts as the reluctant or exasperated participant, though she slowly becomes more involved in their absurd antics.

Despite her typically reserved and sometimes shy nature, Kasumi possesses a surprising capacity for intense anger and violent reactions, particularly when provoked by Olivia's antics or Hanako's schemes. These outbursts feature exaggerated expressions and loud reactions, contributing comedic moments. She also displays unexpected physical strength, evident in activities like arm wrestling. Kasumi secretly writes Boys' Love (BL) stories exploring romantic relationships between male characters, reflecting her discomfort with real-world social interactions, especially around boys. Her nervousness around males contrasts sharply with her fictional romantic explorations. Physically, her more developed figure compared to typical middle schoolers occasionally draws attention or sparks jealousy among peers.

Within the Pastimers Club, Kasumi reveals a pragmatic side, negotiating exchanges such as teaching Japanese pastimes in return for English lessons from Olivia. This arrangement persists even after Olivia's inability to speak English is exposed, forcing Kasumi into increasingly absurd situations. She occasionally breaks the fourth wall, directly addressing the audience about the ridiculousness of the club's activities. Special episodes further highlight her competitive dynamics with Hanako and Olivia, such as participating in an arm-wrestling contest fueled by underhanded tactics to determine the strongest member, maintaining her established traits of reluctance followed by intense engagement throughout their activities.