Madison Keener is David's childhood friend and ex-girlfriend. As said by Eleanor, Madison isn't shallow or materialistic, but has Machiavellian shrewdness. Before David met Julian, Madison was David's only friend in school, a major reason he feels an intense loyalty to her. Although she was in the popular clique, she always gave him her time and attention above others.
Their friendship, which started in first grade, became strained as the years went on, since Madison made it clear in no uncertain terms that she wanted David.
Madison makes a dramatic reappearance in the third chapter of "Inner Magnets". She continues to appear in the quartet until the final volume.