Who is Nick, Karen, Stephanie, Sammy, Petra from The Vintage Book of Contemporary American Short Stories and what is their importance?

Nick is one of the main characters of A Vintage Thunderbird. He is initially dating Karen, who breaks up with him but wants to remain friends and involved somehow; he is interested in a woman named Petra but his involvement with Karen prevents this from happening. He is extremely jealous and not too self-assertive, though he will from time to time pick arguments with Karen.