Walter Mitty's characterization is defined by the authors text structure.
First off, Walter is imaginative. The first piece of evidence to support this is he has five daydreams within the story. All of these daydreams are about him being respected. An example of him being respected is, "I'm not asking you, Lieutenant Berg, said the commander." In this example Walter is a WW1 commander and is leading his men into a storm. This is how Walter Mitty is imaginative.
Secondly, Walter is very childish. The first piece of evidence to support this is that he does not listen to his wife. An example from the story to show this is, "Why don't you wear your gloves? Have you lost your gloves? Walter Mitty reached in a pocket and brought out the gloves. He put them on, but after she had turned and gone into the building and he had driven on to a red light and took them off again." In this section Walter's wife tells him to put on his gloves and then when she leaves he takes them back off. Walter also races his engine. An example is, "He raced the engine a little." In this example Walter is racing his engine at a red light. These are the ways of how Walter is childish.
Finally, Walter is a victim. An example to support this is that a attendant takes his car because Walter is having trouble. An example for the story is, "Hey, better leave the key Oh, said Mitty, handing the man the ignition key. The attendant vaulted into the car." In this part of the story a parking attendant takes Walter's keys and parks for him. A second example is that Walter is guilty in one of his daydreams. An example from the story is, "Mitty let the man have it on the point of the chin. You miserable cur!" In this part Walter is being accused for killing someone with a gun. This is how Walter is a victim in this story.
In conclusion, Walter Mitty is by the authors text structure by him being imaginative, childish, and finally a victim.