Who are the main characters in "The Secret Life of Walter Mitty?"

There are only two main characters in "The Secret Life of Walter Mitty," a middle-aged man and his wife. Walter Mitty himself is the most important character because the story is all about his "secret" fantasies. James Thurber introduces Mrs. Mitty in the opening scenes in order to characterize her, but Mitty drops her off at the beauty parlor in Waterbury, Connecticut, and is alone for much of the story. At the end, his wife reappears and is the same domineering, critical woman she was in the car when she interrupted one of her husband's fantasies by nearly shrieking

“Not so fast! You’re driving too fast!” said Mrs. Mitty. “What are you driving so fast for?”

She is a bossy woman who tends to repeat everything. A moment later she says

“You were up to fifty-five,” she said. “You know I don’t like to go more than forty. You were up to fifty-five.” 

In his fantasy, Mitty must have already gotten the hydroplane up to a hundred miles an hour and was unconsciously pressing harder on the car's accelerator pedal as he forced more speed out of his imaginary hydroplane.

Mrs. Mitty is a realist. Mitty is an idealist. She is an extrovert. He is an introvert. They are entirely different types, but they seem to have a stable marriage because they have adjusted to their incompatibility. She does all the thinking and planning. He obeys orders and keeps his thoughts to himself.

No doubt Walter Mitty was a somewhat exaggerated picture of Thurber himself. He was an eccentric man, and he used his eccentricities in his stories, essays and cartoons. Male characters very similar to Walter Mitty appear in "The Catbird Seat" and "The Unicorn in the Garden." In both of these stories there is a female character who is not too much different from Mrs. Mitty in being demanding, controlling, and insensitive. 

The Mittys may be psychologically incompatible, but they remain married because they are dependent on each other. For example, she apparently doesn't know how to drive a car. They must be New Yorkers who, like so many others, have moved out into the suburbs. Many people who live in Manhattan don't drive cars because the traffic is maddening and there are many other ways of getting around the city. She depends on her husband to drive her into town, and he depends on her to do the planning for both of them. No doubt she tells her husband what suit to wear and what tie to go with it. They would be lost without each other. James Thurber writes about a lonely single man in what his editor Harold Ross called "a mood-type thing" titled "One is a Wanderer." The piece was reprinted in The Thurber Carnival, which is the best collection of his stories, essays, cartoons, and "casuals."

When Mrs. Mitty reappears at the end of the story, her husband is waiting for her in a hotel lobby where he has become lost in his fantasies again. 

“I’ve been looking all over this hotel for you,” said Mrs. Mitty. “Why do you have to hide in this old chair? How did you expect me to find you?”