By definition, the protagonist is the main character. They are the one attempting to affect a change in themselves or the present situation. Then the deuteragonist comes into play. This is the sidekick to the main character. This character supports the protagonist's attempt to bring about the desired results. Next there is the antagonist or tritagonist. This character's main objective is to thwart the progress of the protagonist.
In The Ransom of Red Chief, the story is told from Sam's perspective. He is the one who initiates the action. That would make him the protagonist.
The sidekick to Sam is Bill Driscoll, or Hank as he is called by Red Chief.
Johnny Dorset, or Red Chief, is the antagonist. All his actions result in the downfall of his captor's successful collection of the ransom money that they seek.