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

Diana is the musical's central character and protagonist. As the narrative begins she is in her late thirties or early-to-mid forties, and in the middle of a battle with bipolar disorder, also known as manic depression. She has suffered with this disease, and her family has suffered with its effects, for several years. She has also carried with her the trauma and grief caused by of the sudden death of her infant son almost eighteen years. While the narrative never explicitly makes the connection, it does hint that experiences of psychological illness like Diana's are often triggered by an emotional trauma - in other words, there is the strong sense that Diana's instability developed as the result of her son's death. Despite her illness, and the confusion and uncertainty it triggers in her life and in the world around her, Diana is strong willed and determined, resistant on a deeply profound level to challenges to what she believes is important - in particular, her grief over the loss of her son. Over the course of the narrative, as she goes through a series of medical treatments and a number of confrontations with both her physicians and her family, Diana comes to accept her condition, the loss that triggered that condition, and the need for her to address the grief caused by that loss in order to address the condition. She moves from uncertainty to a greater degree of clarity or, as the final song suggests, from darkness into light.