No not a mirror that you can look into to see the player.
I've got an active that is my mirror, when the player jumps or walks through it I want to create a doppelgänger that moves when I move but in the opposite direction.
In the image below I've got a static active duplicate of the player, I've not yet added any conditions to get it moving because I'm not sure how. I used David Newton's platform engine tutorial to build my game around. The darker grey player on the right needs to move in tandem with the real player on the left, jumping when I jump etc. As it's a mirror image it needs to walk right when I walk left and vice versa.
For those of a certain age (like me) may recall the original Prince of Persia on the Amiga computer. That's the effect I'm going for.
Please login to see this link.