In The Great Gatsby, Meyer Wolfsheim is Jay Gatsby's mentor, father-figure, accomplice, business partner, a friend, and one who eulogizes Gatsby after his death. Really, "Meyer" is an alias that Fitzgerald uses for Arnold Rothstein, the famous gangster who fixed the 1919 World Series.
Meyer, in chapter 4, helps to legitimize Gatsby to Nick during lunch. He says:
‘There’s the kind of man you’d like to take home and introduce to your mother and sister.’.” He paused. “I see you’re looking at my cuff buttons.”
So, Meyer, like Dan Cody (in the middle of the book) and Henry Gatz (at the end) is a father-figure to Gatsby--possibly the most important of all since he helped Gatsby with his "gonnections" to the underworld. In other words, he helped Gatsby become great (he helps him amass his fortune through gambling).
Meyer also serves as an overt criminal, someone who brags about his human molar cuff-buttons:
“Finest specimens of human molars...”
In The Great Gatsby, you are what you wear: clothing reveals character and wealth. Here, Meyer's cuff-buttons show his power, authority, ruthlessness, and his extravagant tastes. In a novel where Daisy cries over Gatsby's silk shirts and Tom shows off his superiority in his polo outfit, Meyer's cuff-links are a grotesque reminder of his corruption. They should signal warning to Nick; however, Nick is strangely attracted to his power.
In the end, Meyer sends Nick a note about Gatsby's death. Tom, Daisy, and Jordan send nothing. Meyer's notes shows, at the very least, his compassion:
DEAR MR. CARRAWAY. This has been one of the most terrible shocks of my life to me[.] I hardly can believe it that it is true at all. Such a mad act as that man did should make us all think. I cannot come down now as I am tied up in some very important business and cannot get mixed up in this thing now. If there is anything I can do a little later let me know in a letter by Edgar. I hardly know where I am when I hear about a thing like this and am completely knocked down and out.
Yours truly MEYER WOLFSHIEM
Remember, it is Meyer's chauffeur who finds Gatsby's body, and it is Meyer's note here that lays his mythological story to rest.
Meyer Wolfsheim is one of my favorite characters in the entire book. While on the outside he is a bit crude, loves to have a good time, and a gambler, on the inside he is much more than that. He is one of Gatsby's most trusted friends, and perhaps the one who led Gatsby to all that he had achieved.