Wendell's the guy who holds the power to make or break Marcelo in Arturo's eyes. He wants something from Marcelo—namely, Jasmine—and spends most of the book trying to manipulate Marcelo into luring Jasmine onto his yacht. When he finds out that Marcelo has "betrayed" him by going camping with Jasmine, he's remorseless about giving Marcelo the letter proving Jasmine fooled around with Arturo. Wendell more or less lives to antagonize.
Of course the argument could be made that the true antagonist in this book is selfishness itself. It's something both Arturo, Wendell, and Mr. Holmes have, and it's something Marcelo must struggle against. Good thing he triumphs in the end, does the right thing, and of course gets the girl.