Also known as the squash-playing tapeworm (okay, we made that up, but it's accurate), Wendell is the archenemy of any person with a heart. He's Stephen Holmes' son, and he's just as corrupt as his father.
He's also a major womanizer, as we see when he asks Marcelo to get Jasmine onto his yacht so he can take advantage of her: "Once she's below deck," he says, "it won't matter what she feels about me. I'll take care of her feelings" (13.38). Ick. So this is a dude whose answer to a woman telling him no is to ignore her completely. Where we come from—actually, where anyone comes from—we call that rape.
Needless to say, Wendell's not a good guy. He attempts to manipulate Marcelo into doing what he wants by pretending to be his friend, telling him, "The bond between our fathers extends to you and me. Keeping that bond, that balance of power, is extremely important. We keep the bond by putting each other first above anyone else" (13.81). But he's not putting Marcelo first when he leaves him stranded at the restaurant and unable to find his way back to work. And thankfully Marcelo can see through his act—for the most part.
When Wendell finds out that Marcelo has gone camping with Jasmine, the bond he pretended to have with Marcelo is broken. He gives Marcelo Jasmine's letter to Arturo acknowledging their indiscretion (seriously, the book is set in 2009, when will these people learn to send and delete emails?). He tells Marcelo it's "the gift of truth," but really he's just trying to hurt the guy's feelings.
It would be nice to say that Wendell changes throughout the course of the book, but he remains a pigeon-kicking jerk. Some people never learn. Wendell's what we call a flat character in literature. He starts out bad, he stays bad, he never changes. When you look at him next to Marcelo and Jasmine, who are all ch-ch-changes all the time, Wendell seems even more gross, pathetic, and, well, tapeworm-y.