Romero, Robert, and Mike act as foils for Jake, emphasizing his fraught relationship with Brett: Romero’s prodigious bullfighting attracts Brett with a level of sexuality that Jake cannot offer due to his impotence, Robert sneaks off with Brett to San Sebastián for a short tryst, and Michael is Brett’s husband-to-be.
To be sure, each character has his shortcomings: Romero is too young, Cohn is viscously insecure and perpetually outcast, and Michael is both bankrupt and quite cruel. In stark contrast to these three, Jake is the only character with a properly emotionally complex and dynamic relationship with Brett, and theirs is the only love that is real in the novel. A central tragedy of the novel is that despite this love they will never be together due to Jake’s impotence. Hemingway sums up this tragic truth in his succinct style in Chapter 19, at the very end of the novel:
"Oh Jake," Brett said, "we could have had such a damned good time together."
Ahead was a mounted policeman in khaki directing traffic. He raised his baton. The car slowed suddenly pressing Brett against me.
"Yes," I said. "Isn't it pretty to think so?"
By using the word "could," Brett affirms the impossibility of the romance, despite Jake being such a good fit. Jake himself confirms this and even enjoys the thought. Therefore, where Brett’s three other suitors are his foils because they emphasize his inability to enter into a physical relationship with Brett, Jake is a foil for them because he emphasizes the qualities that must be present in order for love to grow.