Is this a trick question? For Louise Maillard, you can answer it both ways:
The protagonist Mrs. Maillard is very much indeed a round character in that we see her in one light at the beginning of the story (the docile housewife) and as another (an unfulfilled woman yearning the liberty she once enjoyed) at the end. Her real self unfolds as the story progresses, first adhering to her set role and then not at all. However, there is no place for her at all as a "free" woman. As the title suggests, this was a bitter foretaste of liberty which she never would enjoy at all. The fact that she died at the thought of her husband being alive instead of having been killed is the fatal ironic twist to this tale, taking the reader quite off guard.
Although on the surface Louise Maillard seemed to be the attentive and devoted wife she thought she was, underneath she was rather a stifled person gasping for air. The role she was expected to adhere to in fact didn't suit her at all - so much that the very thought of life without her husband (once past the initial shock at the news of his "death"), sent her imagination soaring. Her disappointment at the reappearance of her husband, who had miraculously escaped any harm, literally killed her. Her need and desire for liberty had been stronger than any love or attachment she felt for her him. She was already this way; only the circumstances of the moment brought it to light. But this is only the case for the reader; stereotype expectation made everyone else in the story think that she had died out of grief for her lost husband instead of for herself. Her "secret" follows her to the tomb.
All the other characters are flat characters as they never change or develop throughout the story line, enhancing in a way the two-dimensional backdrop of her very boring and frustrated existence.