(Oldboy: The more I think about it, the more I think the American remake really didn't understand anything about the original film.)
I'd have to know more about the antagonist to have an opinion on when he should be introduced. Does he already have a defined character arc, or are you speaking more generally?
The first story I thought of when you said mirror was Harry Potter- people talk about Voldemort right from the beginning, but the story sets itself up as though Draco will be Harry's primary antagonist at first.