Among the several characters in "The Story of an Hour," Mrs. Mallard is the only dynamic, round character: a dynamic character usually undergoes some sort of fundamental change as the result of one or more plot elements, and a round character generally exhibits inconsistencies and problems that affect most of us. Dynamic, round characters are often difficult to summarize easily because they are, like many real people, complicated.
In the case of Mrs. Mallard, for example, our first indication of her uniqueness is that she has a bad heart:
Knowing that Mrs. Mallard was afflicted with a heart trouble, great care was taken to break to her as gently as possible the news of her husband's death.
From the first line of the story, we know that we have a character with two serious problems to face--heart trouble and the unexpected death of her husband.
Even Mrs. Mallard's reaction to the news of her husband's death is slightly unusual: instead of "paralyzed inability to accept its significance," we are told that she wept with "wild abandonment in her sister's arms." One could argue that her reaction is unusual because Mrs. Mallard is an unusual woman, a characteristic that takes an even more dramatic turn in the next scene.
After Mrs. Mallard goes upstairs to her room in order to be alone and absorb the news, she was "pressed down by a physical exhaustion that haunted her body," but, at the same time, she is bombarded by life-affirming images--"trees that were all aquiver with the new spring life," "delicious breath of rain," "countless sparrows were twittering in the eaves." These images of life and living take her on a mental path that will lead to a life-altering change in her self-conception.
Something--she doesn't know what--is beginning to intrude on her consciousness:
She was beginning to recognize this thing that was approaching to possess her, and she was trying to beat it back with will . . .a little whispered word escaped her slightly-parted lips.
This leads, of course, to her new belief that she is free of the repression of marriage, free to spend all her days and hours as she wants, free to assert her will in all things for the rest of her life.
The change that has taken here has made her, perhaps for the first time in her married life, a fully-developed, self-assertive individual, the only such character in the story. No other character displays such dynamism, intellectual and spirtual growth, which, by the way, is not a socially acceptable set of changes for a woman of the late 19thC.
One could argue that her death of a heart attack when she realizes Brently Mallard is alive is Mrs. Mallard's way of self-assertion in the face of complete negation of her hopes for the future--in effect, she wills herself to die because, from her new perspective, her life is over.