Who is Mary O'Leary from The Life and Times of the Thunderbolt Kid and what is their importance?

Mary O'Leary was the neighborhood girl that Bill adored as a boy. He thought she was beautiful, and he always liked to land on her if possible during a football game. Bill always wanted to see Mary naked, and one summer, she took off her clothes for the neighborhood boys. She did it, however, on a day that Bill was at his grandparents' so she wouldn't upset him.