What is the author's characterization of Bob in "After Twenty Years" by O. Henry?

The author does not seem either to like or dislike his character Bob. His description of Bob and quotation of his dialogue are objective and neutral, although the reader gets the distinct impression that Bob is a materialistic and unscrupulous opportunist. The reader does not get any impression of how the police officer may be sizing the other man up. The policeman seems objective, like the author. The reader, of course, does not realize that the uniformed beat cop is Jimmy Wells, the man Bob is waiting to meet after twenty years. If O. Henry had adopted a more negative tone in depicting Bob, the reader might infer that this antipathy was a reflection of Jimmy Wells' personal reaction to his old friend. And that might give away the big surprise in the story, which is that Bob is talking to his old friend Jimmy Wells without knowing it.  

Bob is flashy and superficial, but he does not say or do anything to make him seem unlikeable. The reader can actually identify with him because he is being confronted by a cop, "a fine picture of a guardian of the peace"--and most readers have been in that situation at one time or another, even if they have only been stopped for speeding. O. Henry planned to have Bob get busted, but he didn't want his reader to feel glad that this was happening to the fugitive. The reader is more likely to share Bob's feelings of humiliation at letting himself get caught after volunteering so much information about himself and his activities, along with with Bob's feeling of bitter disappointment and chagrin. Bob is depicted as being full of pride and self-confidence at the beginning of the tale and totally deflated and demoralized at the end, in spite of his diamond scarf-pin and his diamond-studded gold watch. The reader can only imagine that Jimmy must feel some remorse about betraying an old friend but satisfaction in having performed his duty "by the book."