Takeshi Aiza is a pianist of Kousei Arima's generation, acclaimed as one of Japan's finest youths through consistent national competition victories and European invitations. His outward rivalry and hatred toward Kousei originate in deep-seated admiration sparked by witnessing Kousei's childhood performances; Takeshi saw him as a piano hero. This complex dynamic stems from their shared history, where Takeshi and fellow pianist Emi Igawa frequently discussed Kousei's seemingly emotionless playing after losing to him, though observers noted both surpassed their peers significantly.

The "Moments" OVA, exploring pre-series events, contextualizes Takeshi's development. He first encounters Kousei during a Towa Hall performance. Initially captivated by Kousei's early emotive style, Takeshi later shifts focus to Kousei's evolution toward technical precision, adopting this as his standard. This admiration fuels Takeshi's drive to surpass Kousei, turning reverence into competition. He actively targets Kousei in contests, aiming to defeat him using the very precision Kousei exemplified. The OVA depicts Takeshi's journey from inspired observer to driven rival, showing how his fixation shapes his musical identity and ambition in their youth.

Progressing into the main timeline, Takeshi faces disillusionment learning of Kousei's psychological struggles and subsequent inability to play. He perceived the "Human Metronome" as an ideal, so Kousei's human vulnerabilities represent a broken pedestal, contrasting with Emi Igawa's continued appreciation for Kousei's earlier emotional expressiveness. Despite this, their rivalry remains impactful. Kousei later acknowledges the musical "sound" born from his competitive interactions with Takeshi was integral to his own artistic revival and growth. Takeshi's presence at Kousei's final performance underscores their enduring connection, symbolizing the culmination of their intertwined journeys as musicians who pushed each other forward.