This is a good question. From the details of the short story, we can say a few things about Della and Jim.
Della was a frugal, careful, responsible person. We know this, because she saved every penny literally. We read this right in the beginning of the story.
"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."
Della also loved Jim very much. As we read the story, we realize that Della had beautiful hair and she gave up this hair to buy Jim a chain for his pocket watch. We can say that Della's love was manifested in her sacrificial giving.
When it comes to Jim's character, the short story does not say much. However, we can say that Jim was a responsible and reliable person. Della does make the comment that he was never late. Most importantly, he was equally a loving person, because he sold his prized possession, his watch, to buy combs for Della hair.