In "The Gift of the Magi", what would be Delia's negative character traits? What about Jim's?
This is an interesting question since Jim and Delia's characters are usually defined by their positive traits. One of the biggest character traits that I would characterize as negative would be a lack of communication.
Each character makes a great sacrifice for the other by selling his or her finest, and only, possession in order to obtain money to buy the other a worthy Christmas gift. In the process, they cancel out each others gifts because now, the purchased gifts have no value because that which they were supposed to complement are gone.
Jim and Delia should have communicated about their desire to purchase each other a worthy Christmas gift, then they would have settled on the fact that they did not need material possessions to make each other happy, they were completely satisfied by their relationship.
The story focuses on the sacrificial nature of their love and the generosity of their hearts.
O. Henry's remark that Jim and Della are "two foolish children" suggests that their faults are those of children: Della is impulsive while also Jim does not think ahead as he forgets to consider that his wife might leave the house and sell her hair. For both these impetuous acts (foolishness), they are forgiven and then termed "the wisest," since in their impetuousness there is love (wisdom), the haste to purchase something in which the beloved will delight.