I would say the biggest aspect of Della's character that emerges from her thoughts, words and actions in this tremendous story is the way in which she loves Jim and is willing to do anything she can to get him a gift that she feels is worthy of that love, even if it involves self-sacrifice. The very first paragraph of the tale makes it abundantly clear how hard she has been trying to save money to buy him a present:
Pennies saved one and two at a time by bulldozing the grocer and the vegetable man and the butched until one's cheeks burned with the silent imputation of parsiony that such close dealing implied.
Thus it is revealed to us that Della has faced shame and censure for Jim, and above all, her action in sacrificing her hair for Jim reinforces her love for him:
So now Della's beautiful hair fell about her rippling and shining like a cascade of brown waters. It reached below her knee and made itself almost a garment for her. And then she did it up again nervously and quickly. Once she faltered for a minute and stood still while a tear or two splashed on the worn red carpet.
Della's willingness to sell her hair, which is clearly her pride and joy, for Jim so that she can buy him a present worthy of her love for him indicates, more than anything else how much she cares for and loves her husband.