Kappei Sakamoto attends Seirin High School as a high school student, possessing extraordinary athletic abilities across numerous sports despite his extremely short stature of under one meter tall. His black hair and brown eyes are frequently animated by energetic, exaggerated expressions during comedic or intense situations.

His grandfather subjected him to strict Spartan-style training from an early age, forging superhuman physical prowess. The regimen's severity led Kappei's parents to fake his death for protection, causing him to live alone before moving in with Akane Aki's family after his parents relocated to the United States. Kappei and his parents later jokingly likened these hardships to "The Little Match Girl," intentionally exaggerating their past.

Kappei harbors a distinct obsession with white female undergarments, viewing them as a "symbol of purity." This fixation motivates actions like joining the basketball team upon learning the coach favors white underwear. His attempts to peek at underwear often trigger physical confrontations, especially with the hot-tempered coach Kaori Natsu. Despite these tendencies, he exhibits genuine romantic feelings for classmate and assistant coach Akane Aki. Their relationship evolves from initial strangeness to mutual affection, though Kappei displays extreme jealousy toward others interested in her. He also competes for her attention with her dog Seiichiro, who resents him and schemes against him.

Academically, Kappei struggles intensely and cheats during exams, starkly contrasting his sports brilliance. His athletic talents span basketball, table tennis, fencing, sumo, triathlons, golf, ice hockey, and more. He employs absurd techniques in competitions—temporarily disappearing, using his uniform as a parachute, or floating mid-air. These antics, combined with impulsive, attention-seeking behavior, frustrate teammates and rivals. Yet his clever strategies and relentless determination consistently secure team victories, earning popularity among female classmates despite his quirks.

Kappei experiences moments of introspection and growth, particularly regarding his bond with Akane. His rivalry with Seiichiro shifts to an uneasy alliance during challenges, and he reconciles with his grandfather after defeating him in basketball. By the series' conclusion, his connection with Akane strengthens, reflecting a more focused commitment to their relationship.