Della is a dynamic character because she undergoes a shift when debating how to find money for a nice gift for Jim. She highly values her hair, but ultimately decides to sell it to get the money for Jim’s present:
One dollar and eighty-seven cents. That was all. And sixty cents of it was in pennies. Pennies saved one and two at a time by bulldozing the grocer and the vegetable man and the butcher until one's cheeks burned with the silent imputation of parsimony that such close dealing implied. Three times Della counted it. One dollar and eighty-seven cents. And the next day would be Christmas.
There was clearly nothing to do but flop down on the shabby little couch and howl. So Della did it. Which instigates the moral reflection that life is made up of sobs, sniffles, and smiles, with sniffles predominating . . .
Suddenly she whirled from the window and stood before the glass. Her eyes were shining brilliantly, but her face had lost its color within twenty seconds. Rapidly she pulled down her hair and let it fall to its full length.
This excerpt shows how Della is a round character, with a tender yet determined personality.
Similarly, the reader sees how Jim is a dynamic and round character through his complex and puzzling initial reaction to Della’s haircut. Later in the story, we see his love for Della as well as his understanding of the irony in their gifts.
Jim stopped inside the door, as immovable as a setter at the scent of quail. His eyes were fixed upon Della, and there was an expression in them that she could not read, and it terrified her. It was not anger, nor surprise, nor disapproval, nor horror, nor any of the sentiments that she had been prepared for. He simply stared at her fixedly with that peculiar expression on his face . . .
"Don't make any mistake, Dell," he said, "about me. I don't think there's anything in the way of a haircut or a shave or a shampoo that could make me like my girl any less. But if you'll unwrap that package you may see why you had me going a while at first."
Finally, Madame Sofronie is a flat character whose traits we hardly know. She's a minor character whose only purpose is to complete a plot action. Her role in the story is described in just a few lines:
Where she stopped the sign read: "Mne. Sofronie. Hair Goods of All Kinds." One flight up Della ran, and collected herself, panting. Madame, large, too white, chilly, hardly looked the "Sofronie."
"Will you buy my hair?" asked Della.
"I buy hair," said Madame. "Take yer hat off and let's have a sight at the looks of it."
Down rippled the brown cascade.
"Twenty dollars," said Madame, lifting the mass with a practised hand.
