4. Agent Warren Knox/Jim Tolliver
Warren Knox's "baby face" and his innocent, Midwestern charm belie his true nature – a vicious, unhinged member of the Bureau of Investigation with the myopic goal of taking down Nucky Thompson's operation. He solidified his role as one of Season Four's most diabolical villains in Episode Five, where he tortured Nucky's manservant, Eddie Kessler, to the point that the German native's only chance at peace was to commit suicide. Knox – real name Jim Tolliver – is also enmeshed in a game of one-upmanship with his law-school frenemy, the newly named director of the Bureau, J. Edgar Hoover. Unable to face Hoover empty-handed, Knox blackmails Nucky's brother, Eli, into turning informant – for the price of keeping his son, Willie, out of jail.