Responses
hmm i think you might be onto something with the "atone for his sins" theory. he smiles whenever Incheol grabs his collar/is about to hit him. in fact he smiled even in the first chapter when Incheol finds him all beat up in his own home. Wonder what that's all about. And I don't think we've seen him smile with any of his other opponents or partners at all. Curious and curiouser...
I'm thinking the boxer might have been abused or something in the past, maybe he was told pain and suffering are the only way he can atone for his sins.
Or maybe he has a hard time feeling things like most people, so he goes through intense situations to feel something.