Teacher comes from the Lush Planet. He has told Edwin stories about it for as long as Edwin can remember. He has always told Edwin that he would go there one day, even though it is said that the planet is dead.
Just as it is here on Earth, there are underground wonders on the planet called Grandmother by the people who were living there before the Squares came.
Edwin can't remember anything but hiding underground with Teacher, the man who taught him how to survive alone and undetected for his entire childhood.