Who is the person in the wallpaper? What does she do

According to the narrator, the "person" in the wallpaper is a woman, who creeps about the borders.

And it is like a woman stooping down and creeping about behind that pattern. I don't like it a bit. I wonder—I begin to think—I wish John would take me away from here!

We can interpret that the person is the narrator trapped in her world.