Charlie plays well with others—until they ask him for money. Then he likes to say things like this: "'[G]o to the devil, sir! Beggar, impostor!—never so deceived in a man in my life'" (31, 5). We're not quite sure, but we think Charlie thinks he's a scoundrel who can get one over on the other guy.
At the very least, he sure seems like he's got something up his sleeve when he tries to get Frank drunk and doesn't take more than a few sips himself (red alert: we know better than to trust someone who's really invested in impairing our senses).
What's important to note about this dude is that he goes to a lot of trouble to pretend he's as jovial and humanity-loving as Frank, but really he's a misanthrope at heart.