I originally posted this as Kieran, at RPG-D and at my own Harry Potter RPG's forum. But I thought this would be useful for Harry Potter fanfiction writers in general as well.
There are countless ways you can perfeclly easily bring a muggle main character into any Harry Potter story or RPG & have a whole world of possibilities open for it - even if they don't know or never learn about magic's existence! They can know or learn, of course. Just, they really don't need to in order to be endlessly and colourfully written to a muggle's full potential.