Tatsuko Gakumazawa attends school as one of Illyasviel von Einzbern's classmates and friends, alongside Suzuka Kurihara, Nanaki Moriyama, and Mimi Katsura. Her family operates a traditional martial arts dojo and manages seasonal ventures, including a summer beach house and festival stalls. Despite this heritage, Tatsuko displays minimal athletic ability and consistently ranks as the weakest and least physically capable among her peers. Her lack of coordination and strength is especially noticeable during activities like dodgeball, creating a sharp contrast with her family's expertise.

Physically, Tatsuko sports blonde hair styled in odango buns topped with a prominent ahoge cowlick, and has brown eyes. She is typically seen in her school uniform. Her personality radiates hyperactivity, childishness, and immaturity, often leading her to unintentionally create trouble for others and serving as a recurring source of comedic mishaps within the group. Paradoxically, despite her frailty and clumsiness, she exhibits remarkable resilience, frequently emerging from dangerous incidents—like magical battles or accidents—with only minor injuries, a trait that evolves into another running gag.

Tatsuko holds deep admiration for her physically strong father, Gouto Gakumazawa (the ninth head of their family's martial arts style), and her older brothers, though she shows no personal aptitude for strength. Other characters repeatedly label her perceived "talents" as useless, reinforcing her role as comic relief. Across all appearances in the *Prisma Illya* series—seasons, OVAs, and manga—her character remains static, receiving no significant development or expanded backstory beyond these established traits. She functions primarily as a supporting figure in slice-of-life segments, contributing to school-based humor and group dynamics without involvement in the main magical girl plotlines.