Takeshi Kawai emerges as a talented junior boxer from a privileged background, competing against Ryuuji Takane and regional champions in the Japan National Boxing Championship. His technical prowess shines through a specialized upper jab technique. Beyond the ring, Kawai demonstrates equal skill as a pianist, balancing athletic and artistic disciplines. Initially, he exhibits a win-at-all-costs mentality, bending match rules to secure victory. His early personality, marked by arrogance and aloofness, stems from a codependent and unhealthy dynamic with his sister Takako Kawai, where both seek validation from each other.

During Team Golden Japan Jr.'s formation for international bouts, Kawai evolves significantly. He abandons underhanded tactics and innovatively fuses his musical background with boxing, interpreting fight rhythms through concepts like periods, fanfares, and sforzandos. This synergy sharpens his ability to anticipate opponents and time attacks precisely.

The Ashura story arc triggers a personal crisis when Kawai uncovers his hidden past. Confronted by internal conflict, he temporarily defects to join the antagonistic Ashura Clan, seeking to challenge his own legacy. Former teammates intervene, rescuing him from the clan's influence and strengthening his bonds with Team Japan. Post-rescue, Kawai matures into a calmer, more reliable teammate, shedding his earlier smugness.

In the narrative's conclusion, Kawai stands among Ryuuji Takane's allies during the professional title match against Jesus Christ. Following Ryuuji's victory and subsequent death, Kawai attends the emotional church gathering where Ryuuji's body lies. His journey reflects a transformation from isolated ambition to integrated teamwork, forged through confronting his past and accepting peer support.