Who is Granny from The City of Ember and what is their importance?

Granny is Lina and Poppy's grandmother. She has caring for her granddaughters for a long time, but age has caught up with her, and she is no longer able to care for them very well. With all of the things that Granny can no longer remember, she does remember an old box that is in the closet..... something important. Lina finds the box, but Granny dies before they can leave Ember.