Who is Natalie from Next to Normal and what is their importance?

Natalie is Dan and Diana's daughter, a moody sixteen year old who, as the stage directions put it, is "trying to be perfect. It's not going well." Tired of dealing with her mother's unpredictable volatility, her father's half-hearted affection, and the intense focus that they both place on the memory of their dead son (Natalie's brother), she acts out with anger, frustration, sarcasm, and eventually drug use. She has been, for most of her life, essentially ignored by her parents, but never gives up hoping that they'll actually come around and recognize that she is not only alive (while their son is not) but actually part of their lives. Her relationship with Henry (see below) starts out shaky, with Natalie determined to keep him at a distance - she doesn't, it seems, want to care about anybody. Nor does she think that she can, or should. Over time, however, as she realizes how dangerously like her mother she is, and how much worse she (Natalie) could become, she begins to open herself more to ALL the people around her, not just Henry but also her mother and father. As the show concludes, she hasn't completely moved beyond her moodiness, but she has, like the other characters, come to realize that there is at least the possibility of a more hopeful life ahead.