Completely untrue. Every single boxer in this manga has a distinct personality, side character or otherwise. Every single coach in this manga has a distinct personality, side character or otherwise. Ippo's classmate bully, the main reporter, his mother, Sendo's grandmother, the homeless guy who scams Takamura, even the farmers and hunters we meet throughout, all of them have distinct personalities.
But as soon as it comes to love interests, you could literally swap any of them (except for Kumi and the reporter ) and it wouldn't change anything. Any of their desires or ideas are limited to their designated love interests and no one else. They're not characters, just motivational eyecandy.