Sergey has an undeniable mean streak in him, but shows he has a heart as well.
I think that you need that mean streak in order to be like him though. That probably allows him to be as dominant as he is. I'm sure that he can just separate his work and personal life though. When it comes to boxing, he needs to have anger. When he isn't fighting, he is probably a nice guy.
Kovalev is scary when he gets angry. Look at the fights against Sillakh and Caparello.
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