I think there's more levels than just simply hardcore and casual. I mean a casual is someone who just sees a new Pokemon game on the shelf and is like, ok why not give that one a try. They play through the story maybe beat the Elite four and that's it.
A hardcore fan to me is someone on the competitive scene, their constantly breeding, divising teams, movesets, looking for hidden abilities, asking for this Pokemon, with this nature, IVs in these stats etc. When they go into battles they know what the opponent will do before they do it, because its the same teams played over and over again.
I consider myself in between. I mean I'm here on a Pokemon site, and I follow the news yearly, after I beat the E4 I do try and get some of the post game, and I've tried my hand at battling others a few times, so I'm definitely no casual.
At the same time, I've never completed the dex, i don't take the competitive scene seriously, I get bored of the battle facilities after a while, after 20 years I'm still not 100% on the type matchups, so I'm no hardcore fan either.