The shape on the other side of the mirror is you.
Almost.
When you move, it hesitates for just a fraction of a second, enough for you to notice even if no one else does. Their hair is a slightly different color, falls just slightly differently from your own. They are you, but not quite.
And when you stand, hand pressed to the glass and looking them over, seeking out the tiny differences, you can feel the heat of their hand beneath your own, different from what it would be if it were your own body heat warming the glass. Hotter, somehow - perhaps the heat is caught between your hands.
One day, you know, they will reach through and grip your wrist, and you do not know what will come after. Will you pull them through? Will they swap places with you, and leave you to forever trail just behind their actions? Will they pull you through and leave two reflections without sources in the mirror?
You don't know.