Takeshi assumes the Pewter Gym leadership in Kanto, specializing in Rock-type Pokémon. He takes this role to support his nine siblings following his father's departure. After battling Ash, Takeshi grants him the Boulder Badge despite Ash's doubts, then leaves the Gym to chase his dream of becoming a Pokémon Breeder, joining Ash's journey. During the Orange Islands arc, he temporarily departs to work with Professor Felina Ivy before rejoining Ash and Misty for their travels through Johto, Hoenn, and Sinnoh. His career goals evolve, shifting focus from top Pokémon Breeder to aspiring Pokémon Doctor.

A recurring characteristic involves frequent romantic advances toward women he meets, often triggering either a companion's ear-pulling or his Croagunk's Poison Jab for comedic interruption. This trait remains consistent throughout his appearances. He leaves the main group after Sinnoh but makes guest returns in later series, including the Alola arc and Ash's journey finale.

In spin-off side stories, Takeshi features in dedicated episodes like "Brock! Save Pewter Gym!", addressing challenges tied to his former Gym. His role across all media remains a supportive figure balancing caregiver duties, Gym leadership, and travel companionship while pursuing evolving professional ambitions. His character design embodies straightforward, easy-to-understand traits intended to resonate with younger audiences.