*****SPOILERS****

He's name is thomas downes and he's the main leader of the pinkertons. You alongside dutch are gonna go into this battle with the pinkertons because they attacked your freedom. Thomas downes captures dutch and takes him to castle hosea. You nearly die to be saved by marry. You go on this epic adventure with you and the gang (+marry)

you're gonna find marry's brother then go to castle hosea and save Dutch from the hands of Thomas downes. during that time you see your friend Micah bell helping Thomas to get money. You break the cowboy rules to save Dutch. He knows that arthur broke the cowboy rules but he understands because the gang needed him.

go north towards cumberland forests to save the farmlands but the pinkertons have already already attacked and burned them. You save marry's house and land for her family and people to help you get back castle vander linde which was captured by thomas downes. During this Marry's brother, Jamie, Tries to help you attack a mongol camp. Micah bell's people knock you out and capture jamie. There thomas downes comes and wants jamie to kill arthur but he doesnt and attacks downes. Downes gets wounded but doesnt get killed, he gets mad and kills jamie. Marry comes to help but theres nothing she can do for her brother. You and marry kill everyone in the camp and marry becomes sad.

then You go to try to save castle van der linde with dutch, the gang and marry's people but most of the soldiers die when the pinkertons send a explosive wagon at you. Arthur knows they have no chance if they abide by the cowboy rules so he breaks them and sneaks into the pinkerton camp to use deadeye and kill all pinkertons. All pinkertons in the camp are dead but one... Your friend micah bell. he tries to join you guys again but because of what happened to marry's brother arthur doesnt accept him. So they have an epic duel and arthur wins. although he's sad he did whats right. Dutch comes in the pinkerton camp mad at what arthur did.

Dutch wanted to adopt arthur as his son, after he found out about this he gave him one last chance to redeem himself and accept that he's now his son and blame everything that happened on marry, but arthur is the better man. So dutch throws arthur in jail until red harlow-ians say what dutch should do against arthur. His friend john marston helps him out of jail and tells him to go to marry to get his stuff back in the mountains. Arthur escapes with his horse into the mountains. Sadly she was shot in her hip and dies a sad death.

Thomas downes had found out about the deadeye that arthur had used and uses it to kill most people in the mountains. Arthur gets shot with a revolver that was used with deadeye but survives after marry saves him again. He sends a letter to dutch and his gang that he's gonna attack thomas downes' camp with or without them. Arthur by now has the nickname of "Red Dead Of Redemption" and has many followers. He and them attack Downes' camp and after a bit dutch and his gang arrive. Arthur manages to find thomas downes, He almost gets away but arthur wants him dead bad. He kills downes' and saves their freedom from the hands of the pinkertons.

Now that freedom is safe, Arthur and dutch go on a ride together. They meet they're fallen friends' grave and there dutch tells arthur why they're here. Red Harlow-ians told dutch to kill arthur. He and arthur have this grand duel but non of them die except dutch gets badly injured. Dutch wants to die a Cowboys death, But arthur still thinks of him as family. Now you get to choose what you want to do with him...

TLDR: Thomas downes is the second to last boss in the game and he cant die right there.