Gigi Sullivan, originally named Kappei Sakamoto in Japanese media, is an American teenager sent by his parents to study at Tokyo's Seirin High School to curb his rebellious and undisciplined behavior. They specifically aimed to separate him from his grandfather, who enforced intense childhood basketball training using a stick. He lives with Anna Akane, a kind-hearted student assistant coach for the school's basketball team.

Physically, Gigi is extremely short, frequently noted as under one meter tall, creating a stark contrast with his extraordinary athletic talents. He demonstrates exceptional skill across diverse sports including basketball, table tennis, fencing, surfing, ice hockey, golf, and marathon running. In his primary sport, basketball, he executes acrobatic moves like prodigious leaps and dunks, employing unconventional techniques such as inflating his trousers for extra lift. Despite initially disregarding rules and structured training, his natural aptitude lets him excel effortlessly, often outperforming rivals undergoing strenuous workouts.

Gigi exhibits a distinct obsession with white female underwear, particularly Anna's, viewing it as a "symbol of purity." This fixation drives repeated attempts to look under girls' skirts, especially Anna's, resulting in frequent comedic confrontations and disciplinary actions. His infatuation with Anna fuels much of his motivation, though he contends with a unique rival for her affection: her dog Salomone. Salomone communicates solely with Gigi and Anna, frequently sabotaging Gigi's efforts to get closer to Anna through elaborate pranks and traps, creating a contentious yet sometimes cooperative dynamic.

Throughout his story, Gigi evolves from a disruptive individual into a committed team member. Initially resistant to joining formal sports clubs, he eventually leads the basketball team to tournament victories, including a decisive match against a team coached by his estranged grandfather—a figure dressed as Darth Vader who initially seeks revenge but later reconciles with the family. His athletic successes extend beyond basketball as he competes in various interschool events, defeating opponents with specialized traits like hypnotic powers or sticky sweat.

A significant development occurs when Seirin High faces demolition for a funfair project. Gigi participates in a "superman competition" against elite athletes to save the school, contributing to his team's victory through resourcefulness and skill. This achievement solidifies his relationship with Anna, leading to their engagement by the series' conclusion. His eccentric rural American parents, uncomfortable in urban Tokyo, occasionally appear, adding comedic elements through their misadventures.