Who helped Holmes in many of his cases?
I'm assuming that this question is as obvious as it sounds. Dr. John Watson, Holmes' sidekick, medical adviser, and security backup, ever-ready with his Army service revolver, is the main assistance for Holmes. Inspector Lestrade of Scotland Yard makes appearances in some stories, but not this one. To say that Watson "helps" Holmes is a bit misleading; most of the time Holmes does the solving, and Watson isn't let in on the solution of the crime until the end, like the reader.
"The Red-Headed League" is no different. As soon as Holmes hears the description of the scar on the forehead of Jabez Wilson's assistant "Vincent Spaulding", he knows him to be a notorious criminal named John Clay. From that moment the central mystery is understood by Holmes, for he knows that Clay robs banks. So if Jabez Wilson is being lured from his home and shop for many hours every day, Holmes instantly reasons that it is because Clay is doing something in that shop to enable him to rob a bank. He is, of course, correct. Clay and his gang are tunneling to the City and Suburban Bank vault, which lies behind Wilson's shop.
Watson isn't brought into this until Sherlock is relatively certain of Clay and his schemes. Unlike Holmes, Watson doesn't have a long memory for the idiosyncrasies of famous criminals. A member of the police, Peter Jones, does aid Holmes in the ultimate physical capture of John Clay, but the deduction which made clear the mystery was entirely Holmes'.