David is in every chapter and narrates every chapter. He's the character who suffers the kidnapping of the title, and it's his quest for an inheritance that sets the novel in motion. While Alan is one of the more memorable characters of the novel, appearing in roughly half the chapters, his story starts before the beginning of Kidnapped and is allowed to drift off into nothing by the end. Does he make it to France? We don't know. Does Davie get his inheritance? Heck, yeah! There's no contest: it's pretty clear that Davie is the main character.