Of course, sports anime often feature a variety of characters, both from the main character's team and opposing teams. While many times the main character is among the most beloved characters, some fan-favorite characters even come from supporting characters and opposing teams, most notably selected for just how beloved they are by fans.
Toru Oikawa is from an opposing team in Haikyu!!, but is still a fan-favorite for his entertaining personality. He comes across as harsh and narcissistic, and it's not an untrue description of him. However, fans learn he has a softer side that just wants to do his best, which helps with fans' admiration.
Overall, Oikawa is a character that fans hate to love but is undoubtedly a fan-favorite. Whether it's because fans love his standoffish, narcissistic personality or just because of the deeper implications of his backstory, he remains a fan-favorite for many.