USA has a few... issues. He has multiple personality disorder, but sticks to mainly two personalities.
One of them is a cheerful, love the world, loveable, say hey if you're gay kinda guy. He calls himself America, or 'Murica if he's making a joke. He loves pulling innocent pranks and likes to flirt. He is rarely angry and laughs everything off. He is kind to pretty much everyone, although he simply tolerates Russia, China, North Korea, etc. He likes to annoy them, but more so like a silly child rather than a cruel adult.
His other main personality is not so nice. He refers to himself as United States and god forbid you call him any different. He is very distinct from America, down to the way he walks. He has a very awkward military march. He rarely speaks and when he does he is completely devoid of emotion. He has calm anger. He won't yell at you, but you know that he will kill you. He is violent and cruel. He never laughs and if he is smiling? Run. He stabs and shoots smaller nations for the hell of it. Small? Shot. Socialist? Shot. Gay? Shot. Breathing in a way he doesn't like? Shot. This man will shoot your dog and when you call the police he'll shoot them too. He also gets along with Third Reich and Fascist Italy disturbingly well.
Of course these two are not his only personalities. He has others that represent his states. Usually its just the majorly distinct states like Texas and New York and his regions like Midwest. When things are really split all of his states will be personified through him. He will often be caught yelling at himself. Most nations have learned to ignore him when he's like this.