In what sense does the storm act as a character in Chopin's story "The Storm?"
In "The Storm," the storm itself could be interpreted as a character if the reader personifies the storm as such. In this interpretation, the personified storm comments on, or supplements, the passion between Calixta and Alcée as they make love. In other words, the storm rises, climaxes, and abates. This rising action parallels their lovemaking.
On the other hand, within the context of the story, the storm is presented as a metaphor; not necessarily as a character. Although both are married and adultery is often considered immoral, the comparison with the storm suggests that their passion for each other is natural. This is not an overt approval of adultery. Rather, it is an acknowledgment of natural passion. In fact, after Alcée tells her there is nothing to fear (of the storm and any guilt of succumbing to her passion), she releases any fear and is free enough to embrace him and even laugh.
They did not heed the crashing torrents, and the roar of the elements made her laugh as she lay in his arms.
Note that although there is deception (Calixta and Alcée never tell their spouses of the affair), the end result was as natural as a shift in the weather. "So the storm passed and every one was happy." In this story, both the storm and passion are natural occurrences and in certain (or most) circumstances, they should not be feared.