To me the definition for a "Hardcore gamer" would be someone who spends a great deal of time playing game(s) with extensive knowledge of said game(s). A
casual for me, simply put, would be someone with limited time to play.
Jan-Poo: I used to be like that back in middle/high school when i had LOTS of free time
but nothing out of the ordinary spending ungodly hours in that game
but dang how i miss it, been 3 to 4 years since i left, no longer had the time for it.
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--A dream called "Youth". We'll eventually wake up from this dream, but the memories will continue to endure. - Karasuma, School Rumble
--Today you are You, that is truer than true. There is no one alive who is Youer than You. - Dr. Seuss