Personally speaking, a hardcore fan is someone who's a part of the franchise/media, of which he's a hardcore fan of, in every way. He/she keeps himself/herself well updated. They like to talk about it, passionately. It can be said, in a way, they are one who "love" the media.
I'd also like to mention that there's a thin line between being a hardcore fan and being a fanatic/fanboy/fangirl. When the "love" changes to "obsession", a hardcore fan changes to a fanatic/fanboy/fangirl. They are so obsessed that they find it hard to digest any kind of negativity against their franchise/character/whatever they're fanatic of.