Granny is a source of constant comfort and encouragement for Mark. When things get bad at home, her house is a refuge. She believes Mark has what it takes to succeed in the world, and her belief in him keeps him going.
It is also through Granny that Mark is introduced to tennis – it was at the home of Granny's employer (the Smiths) that Mark is given his first tennis racket. Throughout his life, Granny consistently tries to set a good example for Mark and attempts to help him make connections and meet people who can provide opportunities for him.