Who is Abby and William Black from Extremely Loud and Incredibly Close and what is their importance?

The second of the Blacks that Oskar Schell interviews, Abby is a 48-year-old epidemiologist living in a townhouse of Bedford St. She is a beautiful, large-breasted, unsmiling woman for whom Oskar wants to invent something to make her like him. She laughs at his corny joke about her beauty and says she does not know his Dad. Something about the way she says it makes Oskar suspect she is withholding something. Claiming thirst and diabetes, Oskar gets in the door, talks about a variety of subjects, and is turned down for a kiss. Remembering something that could help Oskar on his quest, Abby phones his house after he leaves and talks to his Mom, alerting her to the quest.