Takeshi Kawai, a talented boxer and pianist from Niigata Prefecture, began his boxing career under the strict tutelage of his older sister Takako. Her intensive childhood training forged a deeply codependent bond between them.

He rose to prominence as Niigata's representative in the Champion Carnival tournament, where he initially exhibited overconfidence, particularly underestimating competitor Ryuji Takane. His defeat by Ryuji in the finals sparked newfound respect and eventual camaraderie. Kawai's boxing style specializes in upward-cutting punches, including the Right Upper and advanced techniques like Jet Upper, Jet Lavender, and Jet Scramble. He later joined the Golden Japan Junior team alongside Ryuji, Jun Kenzaki, Ishimatsu Katori, and Kazuki Shinatora.

A pivotal moment occurred when Takako chose to leave him temporarily during the Nichibei Kessen (Japan vs. North America) tournament. This forced separation spurred increased self-reliance and maturity, as he navigated competition without her guidance. He contributed decisively to Japan's victory over North America by defeating opponent Missie Charnel.

He participated in the Sekai Taikai (World Tournament), facing international teams. During this event, he briefly joined the antagonistic Asura clan before returning to the Golden Japan Junior team. His final match secured Japan's world championship title with a flawless victory against Orpheus of the Greece Junior team.

Following this achievement, he retired from boxing alongside his teammates. In later years, he became a mentor to Rindo Kenzaki, the protagonist of the sequel series, passing on his knowledge.

His appearance features long black hair (depicted as blonde in manga) and gray-blue eyes. He typically wears traditional Japanese attire resembling a shinobi robe, while his boxing uniform consists of a white singlet with Japan's flag and white shorts with violet accents during international competitions.