Whatever the game you are playing, the High Concept is a core aspect of any character. It speaks to who the character is deep inside. In essence, it can be seen as something of the "trope" that the Player is looking to fit into, while the other aspects or things done with a character refine that trope into a truly original character.
As I work towards making a hack for FATE: Superheroes (not that there aren't already tons out there), I find myself trying to go through and code a character up as I play around with the rules. The fun part is, this character to me has effectively become a concept themself in my head.
The Birds: Joking
14 hours ago