The protagonist in a drama is always the character who initiates the conflict. In "The Bride Comes to Yellow Sky," it would have to be "Scratchy Wilson" who is the protagonist, since there was is no conflict, in fact utter boredom, before he appears on the scene. He has had a long-standing feud with the town marshall of Yellow Sky, Jack Potter. Therefore, Jack is the antagonist. The most recent episode in their ongoing conflict has been delayed because Jack was getting married in San Antonio, but Scratchy goes to his house looking for him, terrorizing everybody else on his way there, including the dog.
The reader is led to expect a violent confrontation between the two men if and when they run into each other.
Potter was about to raise a finger to point the first appearance of the new home when, as they circled the corner, they came face to face with a man in a maroon-colored shirt who was feverishly pushing cartridges into a large revolver.
Scratchy is appalled when he finally comprehends that Jack is not wearing a gun and that he is bringing his new bride home with him.
"Well, said Wilson at last, slowly. "I s'pose it's all off now."
"It's all off if you say so, Scratchy. You know I didn't make the trouble."
Jack statement that he did not start their feud is further evidence that Scratchy is the protagonist, because he was the man who initiated the conflict a long time ago. A protagonist typically has a strong motivation which carries the plot forward to a conclusion. Scratchy does not have a strong motivation. He just gets ornery when he gets drunk. There is no strong motivation because there is no "MacGuffin," no bone of contention, nothing tangible for the two men to be fighting about. The fact that Scratchy's motivation is weak explains why the conflict evaporates so abruptly.
Scratchy will be a better citizen now that his ongoing conflict with the town marshall has been resolved. This illustrates the main theme of Stephen Crane's story, which is that it was the arrival of women and children that tamed the Wild West.