This character is her grandma. Her grandma lets slip all these secrets of adulthood that help her become the lady she is. She tells about her life as a younger women. She told Marjy "the first marriage is practice for the second" which is what I think Marjy shaped her life over. Second chances are what got her through all the troubles.
Theres a part of the book where they have to run up to Marjy's apartment to poor all of her dad's illegal alcohol down the toilet. Her grandma reveals how she used to have to do this for her husband. I think Marjy aspired to be her grandmother when she grew up, because she did a bunch of rebellious stuff in her adolescence.
Although Marjy has a mother, I think her grandma is the supporting women figure in her life. She shares secrets with her like a best friend, learns stuff from her, and basically treats her like an idol. She shows how she loves her a lot. Her grandma is protective but educating, and Marjy almost worships her with respect and love.
