Why Cohn?

At the beginning of the The Sun Also Rises, we are introduced to one Robert Cohn, and, at first glance, it is not clear why. Cohn is an important character, certainly, but he is not the protagonist. I remember the first time I read The Sun Also Rises, I was annoyed that Cohn hadn't waited his turn to be introduced at the right time. The other characters are introduced later, including some very important ones, like Brett, and it's not immediately clear why Cohn gets preference over Jake himself.

I can offer two theories. The first is based in the fact that Jake is a very passive character. The closest thing he has to a "motive" or "agency" is his determination to go the bullfight. He loves Brett but doesn't do anything about it; he wants to find meaning in life but doesn't know how to go about that, either, and he prefers to live vicariously through bullfighters. All in all, not the most dynamic individual. Beginning the book with a description of Cohn makes sense, because Cohn does have agency and he does try to get what he wants. When he falls in love with Brett, he refuses to leave her alone until she reciprocates his affection. When it becomes clear that she'll never love him, he leaves. Cohn is everything that Jake should be, or would like to be. I wouldn't go so far as to say that he's the main character of the book--he isn't--but it makes sense that he's capable of wresting the opening of the book away from Jake, who has less agency.

My second theory is that Hemingway is trying to establish a link between Jake and Cohn. He is trying to make it clear that Cohn is more important than we think, that we should carefully consider the narrator's relationship with Cohn, etc. Why would he do this? Well, Jake and Cohn are similar in one very important way: namely, they fall in love with the same woman. They handle the situation differently--Cohn pursues her ardently, to the point of embarrassing himself, while Jake tries to suppress his love when Brett tells him that they can't be together. To belabor the point with an unnecessary but apt poker metaphor, Jake folds while Cohn goes all-in. So although Cohn is not the main character, he is an analogue to the main character, which makes him more important than he might otherwise be.