Who is Mary Lisbon from The Virgin Suicides and what is their importance?

Mary Lisbon is sixteen-years-old. She is poised, polite and very self-conscious. She spends a great deal of time in front of the mirror, taking care of her hair and polishing her looks. Her blonde hair is noticeably darker than that of her sisters. She's also has a slight mustache which she's been trying her best to hide. She pays attention to her appearance and wears a brightly colored sweater every time she comes out of the house to pick up the mail, even though everything around her seems to fall to colorless ruins.