Who is Mary from Earth Abides and what is their importance?

Mary is Ish and Em's oldest daughter and the first to marry. She weds Ralph, the son of Molly who is one of Ezra's wives. Ish notes that Mary and Ralph would have previously been considered too young, but in the new culture it's accepted and encouraged to marry early. Ish says that Mary becomes stolid with age and with each child she produces. While he continues to value intellect, he says that could actually be a very good thing and that people like Mary would likely do well in this new culture.