To be honest, Warren is kind of a cipher. He's a combination of generic hot guy and boy next door; he sparks the jealous conflict between Marjorie and Bernice, but otherwise doesn't do a whole lot. Interestingly, he's the one object of desire in this story, which is one of the interesting quirks of this story. Instead of presenting us with a more traditional idealized woman, Fitzgerald instead shows us what women want (or at least, what he thought women wanted) in Warren, a handsome, suave Ivy League guy.
He seems like a good enough guy – he's always been devoted to Marjorie, despite the fact that she leads him on shamelessly. Frankly, we don't know enough about him to like or dislike him, and, even if we did, it would be kind of irrelevant. What we do know is that Bernice likes him (a lot) and Marjorie likes him (a little) when it's convenient. And he likes both of them. Don't forget that bit.