Davie Balfour is both the narrator and the central character. The great thing about the story being in first person (using "I") is that you feel caught up in Davie's experiences, like you're really sitting and listening to him telling you his story. Combine his first person narrative with the many dates, historical figures, and real places that Stevenson uses in Kidnapped, and you may start to experience a kind of reality effect: despite all the crazy shipwrecks and things, maybe this really happened. (Alas, no.)
In Davie's "Character Analysis" we also talk about the limitation of making one person both the narrator and the main character. We get no alternative perspective on Davie's action or behavior; everything that happens is seen through his eyes. We're always left to wonder what other characters might be thinking – especially Alan Breck, the novel's second-most-important character.