Who is The Oh Mailman Lady from Loser and what is their importance?

The Oh Mailman Lady is an old woman who lives on Willow Street, a few blocks from Zinkoff's house. Relying on a walker, she moves very slowly. Zinkoff never learns her name and during the course of the novel, he switches from calling her the lady with the walker to the Oh Mailman Lady. Zinkoff first met her when his father took him on a pretend mail delivery run on a Sunday and always after that, the old lady calls out, "Oh, mailman," whenever she sees Zinkoff. The Oh Mailman Lady has watery eyes, white hair, and skin so thin and pale it seems almost transparent to Zinkoff.