The two brothers are obviously very important characters. Eli Sisters is one of them. He's a more sensitive and introspective character compared to his brother. Charlie Sisters is the other, more hot - headed. Another important character could be the person they are sent to kill or track down. This character drives much of the plot as the brothers' journey is centered around this mission.
The Sisters brothers themselves, Eli and Charlie, are crucial. Eli is often the voice of reason between the two. He has a conscience that is constantly at war with the violent nature of their job. Charlie, on the other hand, is more impulsive. Then there are the various characters they encounter on their journey in the historical west. These side characters add color and context to the story, like the people in the towns they pass through or the other outlaws they cross paths with.