Kentaro Yamanaka is the star pitcher for Bunan High School's baseball team and a third-year student. His position establishes him as a rival to Seishuu High, attended by the main characters. Kentaro develops a romantic interest in Nakao Yuri, the manager of Seishuu's baseball team. This directly conflicts with protagonist Niimi Katsuya, who also harbors feelings for Yuri, forming a significant love triangle.

Kentaro has a younger brother, Jirou Yamanaka. Jirou joins the Seishuu baseball team during his second year and subsequently develops feelings for track athlete Yasuda Yukimi. Yukimi had previously pursued Katsuya, further intertwining the relationships between the Yamanaka family and Seishuu's students.

While Kentaro's pursuit of Yuri contributes to romantic complications, his character receives less focus in "Nine 2: Koibito Sengen" compared to the first installment. Narrative emphasis shifts toward resolving the dynamic between Katsuya, Yukimi, and Jirou, reducing Kentaro's prominence as an active romantic rival despite his established interest in Yuri.