What are the roles of Robert Cohn in The Sun Also Rises?

Robert Cohn is the outsider of Ernest Hemingway's
The Sun Also Rises. He is scorned by the other characters because
he is Jewish and wealthy, wealth being something that Jake and his friends frown upon.
An old friend of Jake's, Cohn is a former college boxing champion who attempts to bully
his way into Jake's inner circle, but he instead encounters anti-Semitic barbs,
jealousy, hatred and abuse. He even has a short affair with Lady Brett, but when Cohn
continues to pursue her, Brett decides that "he is not one of us." The insecure Cohn is
a romantic, another strike against him in the eyes of Jake and his "lost generation"
pals, and his idealistic American values widely separates him from the
others.