How does Bob describe Jimmy's strengths and weaknesses?
Bob's descriptions of Jimmy turn out to be ironic because, although he doesn't know it, he is describing Jimmy to Jimmy's face. Speaking to the patrolman, whom he doesn't know is Jimmy Wells, Bob says that Jimmy was his "best chum" growing up, and he considers him the "finest chap in the world." These are general positive descriptions showing that Jimmy was a good friend to Bob. Bob goes on to deliver some veiled criticisms of Jimmy, though. He insinuates that Jimmy's loyalty to his hometown of New York was naive. He seems to doubt that Jimmy will have become as financially successful as he has because, in his words, Jimmy was "kind of a plodder." This means Jimmy didn't have drive and competitiveness; by staying in New York rather than going out West, Jimmy has become a run-of-the-mill kind of guy in Bob's estimation. Bob assumes he has gotten "in a groove."
Notwithstanding these perceived shortcomings, Jimmy has strengths, according to Bob. Bob says he "always was the truest, staunchest old chap in the world." Although "true" in this context indicates loyalty, it also implies a level of integrity and commitment to what is true and right. "Staunch" also speaks of loyalty. Bob is convinced Jimmy will never forget about their meeting; he is reliable and can be depended upon to keep a promise.
It turns out Bob understands his friend's character well. The qualities that Bob relates about Jimmy are the qualities that make it necessary for him to turn Bob over to the law. His plodding and getting into a groove in New York City have allowed him to become a consummate police officer, having the ability to withhold his identity from Bob and pull off a sting operation under the nose of a man who has been able to "compete with some of the sharpest wits going." Jimmy remains true not only to his promise to his friend but also to the requirements of the law and society, delivering Bob into the hands of the Chicago police.