On the surface this looks like a film about boxing. It's actually about a love triangle between three people who all hate themselves on different levels.
At school Tsuda and Kojima were friends. When a girl in their class was murdered by a gang who wanted to rape her they swore to get revenge. Kojima trained to become a boxer so he could beat up the gang, but Tsuda didn't make any efforts. The police arrested the gang before they could get revenge. Kojima blamed Tsuda for their failure. After school they never spoke any more.
Ten years later they meet again by chance. Tsuda is a meek insurance salesman who lives with his fiancee Hizuru. Kojima lives in a small, barely furnished apartment. Kojima has become a skilled professional boxer, but he's unable to win fights. When he has the advantage he holds back instead of delivering the knockout punch. It's like he wants to be punished for the failures in his past.
Kojima flirts with Hizuru, but she rejects his advances. Then he taunts Tsuda by telling him he's had sex with her. Tsuda tries to beat Kojima up, but he doesn't stand a chance. Hizuru despises Tsuda for losing the fight, and she despises him even more for attempting the fight. After a series of arguments Hizuru leaves Tsuda and moves in with Kojima.
Has Kojima won the girl? Yes and No. Like in his boxing matches, he's unable to claim his prize. They live together, but they never have sex.
This isn't an attractive film. It's an ugly film. By the end all three characters have destroyed themselves, physically and mentally. I can appreciate that it's a good film, but it's very difficult to watch.