No, John and Arthur Marston are two separate entities in the Red Dead Redemption 2 Epilogue. While they do share some similarities in physical appearance, John is a significantly different character.
At a basic level, John looks identical to Arthur in the first Epilogue mission, however, there are several distinct differences that set them apart. For starters, John is a little bit leaner and some of his facial features have changed. He has deep lines on his face from stress and sadness that Arthur did not have before his passing. He also wears a different hat and sports a longer hair style.
Gameplay wise, John handles his weapons differently from Arthur. His movements are slower and more deliberate, indicating his advanced age, and his shooting style is a lot more accurate than Arthur's wild and untrained shooting.
The most significant difference between Arthur andJohn comes from the personalities of the two characters. Arthur was a rough and gruff cowboy-type, while John is much more refined, philosophical and measured in his responses. He also handles missions differently, choosing moral and non-violent options- something Arthur only began to do at the end of the main story.
In summary, John and Arthur are not the same character, but Arthur’s spirit lives on in John in certain ways.
