Ayano Takeda has clearly considered him as an important character to put into the series. In the fanbook’s interview, she sees Euphonium something like a shoujo manga. She has paired off characters with each other: Nozomi with Mizore, Asuka and Kaori, and Kumiko and Shuichi.
Kumiko can be brash with her words and sometimes appear rude. She doesn’t censor herself when she’s in the zone. But she can also be moody and nervous. She needs someone to tell her that’s okay and cool her off. That role is Shuichi.