The central character that you will undoubtedly want to focus on and, indeed, the only character that we gain any real insight to, is that of Mrs. Mallard, who, in the two and a half pages of this excellent short story is the main focus of the tale. The very first sentence informs us that she is "afflicted with a heart trouble," which suggests that she is a fragile person, weak and not robust in her physical frame. However, the way that she greets the news of her husband's death immediately distinguished her from other women, as the author says:
She did not hear the story as many women have heard the same, with a paralysed inability to accept its significance. She wept at once, with sudden, wild abandonment, in her sister's arms. When the storm of grief had spent itself she went away to her room alone. She would have no one follow her.
Clearly, she is a woman who is able to experience emotions and feel them deeply, and is not inhibited in expressing them. She is able to react suddenly and deeply and does not repress what she is feeling.
Physically, we are told that she is "young, with a fair, calm face, whose lines bespoke repression and even a certain strength." This indicates that she feels "repressed" in her marriage, but the "certain strength" indicates deep strength in her character that allows her to face this repression. The sense of repression is strengthened as we see the joy with which she greets her new state as a widow:
There would be no one to live for her during those coming years; she would live for herself. There would be no powerful will bending hers in that blind persistence with which men and women believe they have a right to impose a private will upon a fellow-creature.
It is this "freedom" that she experiences, ever so fleetingly, therefore, that is the cause of her death at the end of the story, as to have savoured such a freedom only to have it rudely snatched back from you was the big shock to her heart that ended her life.