Why should Mrs. Mallard be regarded as a complex and dynamic character?
A complex character in literature can be described as one who has traits that come from both nature and their life experience, and Mrs. Mallard does have both. For example, the narrator describes her as "young, with a fair, calm face, whose lines bespoke repression and even a certain strength." To say that Louise Mallard possesses strength is to point out a trait that seems natural to her; it isn't necessarily a learned trait. However, that she might feel somewhat repressed, or that she represses some of her own feelings or ideas, is a learned trait, something that Louise would have internalized as a result of her own life experience.
Later, when she comes down the steps, "she carried herself unwittingly like a goddess of Victory." Again, such a statement describes a combination of qualities: on the one hand, Louise performs this action "unwittingly," meaning that it is unintended or unplanned (her life experience would have taught her that she ought not to appear to rejoice in the death of her husband). On the other hand, it seems as though her joy is a natural feeling for her to have in this moment; it is natural. Therefore, Louise Mallard could be considered a complex character because of her mix of traits that come both from nature and experience.
Mrs. Mallard is complex because she proves to be multi-dimensional. Upon hearing that her husband has been killed, she cries "with sudden, wild abandonment." This initial reaction suggests that she is a devoted wife. It suggests that she loves her husband deeply and that this news is devastating because it will be hard to live without him. However, over the course of the story, she increasingly thrives in her new independence. She emerges from the initial suggestion of a devoted, dependent wife and becomes fully independent. She changes from a simple, traditional female role into a more modern, freeing way of being. She reveals complexity in her ability to change roles.
She is dynamic because she changes. This is the definition of dynamic: the ability to change. Mrs. Mallard's change is dramatic and liberating. She had once tolerated life, but now she fully embraces it:
She breathed a quick prayer that life might be long. It was only yesterday she had thought with a shudder that life might be long.