Takeshi Aiza

Takeshi has a very honest and straightforward personality. He is very upfront about his rivalry and hatred (at first) for Kousei Arima. This hatred is later revealed to be an admiration of Takeshi, and how he looked up to Kousei as a hero. Takeshi looks up to Kousei to the extent of rebuking Kousei when he did not meet his expectations as a pianist.

Takeshi is one of the best piano players of his generation, as mentioned for being invited to Europe for competitions and winning the majority of recent piano competitions in Japan.

It was shown that prior to the storyline, Takeshi and Emi Igawa are sitting together complaining about the "emotionless" Kousei after losing to him in a piano competition. The rest of the crowd points out that Takeshi and Emi are head and shoulders above the rest of the competition, which makes Kousei all the more invincible for beating the two. In the storyline, it is shown that Takeshi looked up to Kousei as a hero, and constantly practiced in order to surpass Kousei.

(Source: Shigatsu wa Kimi no Uso Wikia)