Who is Louisa from When You Reach Me and what is their importance?

Louisa is Sal's mother. Louisa is also a single mother and she and Miranda's mother became great friends when Miranda and her mother first moved into their apartment. It is for this reason that Miranda and Sal have been best friends since childhood. Louisa works in a nursing home and she often brings lists from The $20,000 Pyramid for Miranda's mother to practice with for her appearance on the show.