The faces are familiar as we've come to know Biden's closest confidants. Except for one, one figure remains elusive: Biden's Chief of Staff, Jeff Zients. A man of immense wealth and power, he is virtually invisible, unknown to most Americans. Yet he holds the keys to Biden's day-to-day meetings and pronouncements. Just who is Jeff Zients? Today, we take an in-depth look at this mercurial figure.