In "The Red-Headed League," Dr. Watson is no longer sharing ooms at Baker Street with his friend Sherlock Holmes. He calls on him and finds him interviewing a man "with fiery red hair" whom Holmes introduces as Mr. Wilson. As usual, Holmes assures his new client he can rely on Watson's discretion.
"This gentleman, Mr. Wilson, has been my partner and helper in many of my most successful cases, and I have no doubt that he will be of the utmost use to me in yours also."
Watson sits down to listen. As is his custom, he tries to make deductions about the newcomer by imitating the methods he has learned from Holmes.
I did not gain very much, however, by my inspection. Our visitor bore every mark of being an average commonplace British tradesman, obese, pompous, and slow....Altogether, look as I would, there was nothing remarkable about the man save his blazing red hair, and the expression of extreme chagrin and discontent upon his features.
So the details Watson seems to regard as the most important are Wilson's red hair and expression of vexation. Watson misses everything Holmes sees and from which he makes deductions. These are Wilson's muscular hands, his snuff-taking, his Freemason's breastpin, his Chinese tattoo, and his worn right cuff. The snuff-taking could apparently deduced from traces of snuff on his vest.
The most significant deduction about Wilson, at least for the reader, is that Wilson has spent some time in China. He has evidently been quite a traveler. No doubt he has visited other countries in the Far East besides China. He probably was employed by some British firm that was engaged in trade with these far-off countries. The fact is only important to the story because it helps explain why Wilson might never have heard about the so-called League of the Red-headed Men. Wilson's new employee explains it to him and pretends that everybody knows about it because it was in all the papers when it was founded by am American millionaire, Ezekiah Hopkins,
"...who left an enormous fortune in the hands of trustees, with instructions to apply the interest to the providing of easy berths to men whose hair is of that colour."
Jabez Wilson also explains to Holmes how it could be that he had heard nothing about this benevolent institution during the time he has again been living in England. Arthur Conan Doyle is taking great pains to make it credible that a man with such unusual red hair should have heard nothing at all about the supposedly well-known Red-Headed League.
"You see, Mr. Holmes, I am a very stay-at-home man, and as my business came to me instead of my having to go to it, I was often weeks on end without putting my foot over the door-mat. In that way I didn't know much of what was going on outside, and I was always glad of a bit of news."
So Wilson was easily taken in by the cunning John Clay, masquerading as Vincent Spaulding. Wilson has been living abroad for years. He probably made enough money to open his small business in London fairly recently. And at the conclusion of the story, while Holmes is explaining his solution to Watson over glasses of whisky and soda, Holmes describes Wilson as "this not over-bright pawnbroker."
Doyle created exactly the character he needed to fit the unusual plot. Wilson is extremely gullible and naive. He is also greedy, which explains why he is so distressed when he loses his sinecure with the Red-Headed League that he comes to Sherlock Holmes hoping for free advice and assistance.