She is a shadow on the wall.
In the corner of your eye, you see her in your peripheral vision. You know she's there, but you can't really see her—there are too many shadows in this world. She is everywhere and nowhere at once, but her presence is felt everywhere.
She has been with us for so long that we forget about her. We forget about how close she is to us, always present but never seen. It's only when we look closely that we realize what she means to us: she is our reflection, and yet she reflects nothing of ourselves back at us. What does this mean?
Who knows? Maybe there's something more to it than just being able to see ourselves reflected in another being's eyes—maybe there's something more to seeing ourselves reflected in someone else's eyes than just being able to see ourselves reflected in another person's eyes...