Tsuyoshi Yamane is a third-grade boy characterized by a long face, a teardrop-shaped nose, and spiky hair. His name, meaning "strong," embodies a personal aspiration toward bravery and resilience, yet his actual nature contrasts sharply with this meaning. Frequent stomachaches plague him due to a sensitive stomach, triggered easily by minor dietary issues or nervousness. This condition causes significant distress and frequently interferes with his daily activities. Despite a persistent desire to project strength and determination, his preoccupation with stomach problems often undermines his efforts, leading to perceived failures. Classmates, including Maruko, sometimes offer support and encouragement during these episodes.

His favorite foods include castella sponge cake, bananas, and tsukudani seaweed boiled in soy sauce. He initially disliked natto fermented soybeans but overcame this aversion with Maruko's assistance. While early anime episodes frequently showed him alongside classmates Nagasawa and Fujiki, later episodes depict him more independently. His character exhibits no documented development or appearances in spin-offs, films, or other extended media beyond the main series.