Thanks!
The idea was that you're being followed by some unknown presence as you're trying to make your way home and that you had to use a mirror to be able to keep track of it. And the Stalker would react to the mirror in a different way than it does now.
I had an idea of how I wanted the Stalker to look, it was gonna have a black robe and a mask (as it was hiding its true self), I looked at the Kaonashi (or No-Face) from Spirited Away as inspiration.