That Jim and Della are simple working class people is revealed in Della's speech: ""Cut it off and sold it," said Della. "Don't you like me just as well, anyhow? I'm me without my hair, ain't I?" That they are young and in love is revealed by the biased limited third-person narrator: "Poor fellow, he was only twenty-two--and to be burdened with a family!" That they are foolish in material ways is clearly stated by the narrator: "two foolish children in a flat who most unwisely sacrificed for each other." That the narrator is representing their material foolishness as spiritual wisdom that reflects the wise sacrifices of the first Christmas Magi is made clear by O. Henry's didactic ending: "these two were the wisest."
Della is an emotional young woman who is maybe not yet twenty, since Jim is but twenty-two. She is demonstrative with her feelings and generous in her love. While she is foolish in her longings and desires for a better life, "her heart had simply craved and yearned over [the combs]," she seems to be wise in her spirit of giving (though if it weren't Christmas, her actions might not have the same effect). She is given to sobbing loudly and letting tears fall on the carpet, and she acts impetuously without thinking things through, such as not thinking about how to apportion the $20 gotten for from her hair to buy a gift and expand their budget.
Jim is a responsible working man, though very young and already with Della to provide for. He is as generously giving and sacrificial as she. He is philosophical about loss; he sat on the sofa with his hands behind his head and suggested they save the gifts for awhile. He bases his love for Della on deep qualities, not on superficialities. Jim seemingly similarly used the whole value received from his watch sale for Della's gift without thought to buying a lesser gift and to expanding their budget. Neither have foresight though both are praised by O. Henry as sacrificially giving and loving.