At first glance, Mrs. Mitty appears to be a good candidate for the role of primary opposition to her husband, Walter Mitty. She nags him constantly about buying overshoes and puppy biscuits, scolds him about driving too fast, and always wants to check his temperature. In fact, she is really quite annoying.
But Mrs. Mitty is only part of a larger picture. The greater antagonist in the story is Walter Mitty's humdrum daily life. He is bored stiff by his normal existence. Running errands, for instance, fails to stimulate his mind and his interest in any way whatsoever. He does not care about buying overshoes and puppy biscuits or taking his wife to her hair appointment or checking in with his doctor. He wants excitement and activity, something that gets his mind going and his heart pumping. But since he cannot find it in the oppressive real world, he makes it up in his daydreams.
Indeed, Walter Mitty is an expert at daydreams. To conquer his boring antagonist, he becomes a fighter pilot, a surgeon, a defendant on trial for murder, and even a man standing bravely before a firing squad. In his own mind, Walter Mitty is a hero, and his imagination brings color and adventure to his life, helping him cope with the boredom of the real world.