Jimmy Wells is a number of things, depending on how you approach the story.
To Bob, Jimmy Wells is a former friend who opted to stay in the east. From Bob's point of view, Jimmy was his best friend and the "best fellow in the world." Bob has fond memories of the two growing up together in New York.
If we look at the story from Jimmy's point of view, however, we see a police officer, an honest citizen, and a compassionate friend. Instead of arresting Bob (his former friend) himself, Jimmy asks another officer to perform his duties on his behalf.
In refusing to arrest Bob, Jimmy Wells preserves whatever goodwill still remains between him and his former friend. In other words, the fond memories from past shared experiences remain—regardless of the developments of the present.
Meanwhile, if we look at Jimmy Wells from the viewpoint of the author, we see him as a dedicated public servant, a man committed to the cause of justice.