"Hard-core" is a way of saying "extreme".
"Hard-core" pornography is the very very unpleasant kind; "hard-core fans" are the ones who will turn up at every single concert on a tour; "hard-core protestors" are the ones who build tree houses and live in them to stop you chopping the trees down.
Some-one who is hard-core at doing something does it to the extreme, or is very dedicated to it in some way. I've seen it start to be used among teenagers to describe people directly, meaning someone prone to extreme or excessive actions or reactions. I've also heard "crazy" and "wild" used the same way.
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