Jim is hard working and dedicated to his family. He doesn't get as much description in the text as Della does, but the little we see of him tells a lot. He is obviously thoughtful and caring since he sells his treasured watch to get Della an elegant set of combs. He is also described as "thin and very serious," which could be interpreted as a result of his stressful money situation. But Jim's main characteristic is that he is loving. He never complains, though he needs new clothes and is barely making ends meet, and he sacrifices his prized possession to buy Della a gift. He always "greatly hugged" his wife when he got home. At the end, he is undisturbed by Della's change in appearance and doesn't seem bothered that the he has no watch to put on the chain Della bought for him. He calmly sits on the couch and asks Della to get dinner, which likely puts her at ease.
You already have an excellent answer by magana584, so I just have a few things to add. In O. Henry's "The Gift of the Magi," James Dillingham Young used to earn thirty dollars a week; however, he now only gets twenty dollars a week, and of course that means he and his wife, Della, must be much more careful with their money. Della loves her husband very much. Every night when he comes home she hugs him, which, the narrator says, "is all very good."
They do not have very many fine things, but one of them is Jim's gold watch. It was handed down from his father and his grandfather, and he is very proud of it.
Had King Solomon been the janitor, with all his treasures piled up in the basement, Jim would have pulled out his watch every time he passed, just to see him pluck at his beard from envy.
When Della finds the perfect fob for Jim's precious watch, she describes it--and her husband--this way:
It was like him. Quietness and value--the description applied to both.
Della is worried about Jim's reaction when he sees that she has cut off her long, beautiful hair. She has no idea, of course, that he has bought her combs with the money he got from selling his watch. When he sees her, he is surprised, naturally, but he is not angry or upset either at her short hair or the fact that he sold his watch for something that turned out to be useless to Della. He rather sees it is a delightful joke and just wants to set both gifts "aside for awhile" and enjoy Christmas with his wife.
From all of this we can characterize Jim as a kind man who loves his wife enough to sacrifice what is most important to him for her happiness. He works hard and has a positive outlook on life; he is also optimistic about their future together.
We know very little about Jim. He only appears towards the end of the story and is not given any physical description, although we can assume that he is young and good-looking enough to make Della love him. The fact that he has been carrying a gold pocket watch suggests that he works in an office and wears a suit and necktie. He is not doing very well in the world of commerce. He had been earning thirty dollars a week, but now he is only earning twenty dollars a week. A young man should be getting raises and promotions, but Jim seems to be moving down the ladder instead of up. This suggests that he is either not very ambitious or not very talented. He shouldn't have to be selling his gold pocket-watch in order to buy his wife a modest Christmas present. And she shouldn't have to be selling her precious hair to buy him a watch-chain. Della thinks that the chain she buys for her husband is like him and like his gold watch: it has "quietness and value." Maybe he is a little too quiet. He may be the kind of man who just does his job conscientiously and never gets noticed or appreciated by his superiors. The fact that he sells his treasured watch shows that he is a loving husband. We suspect that these two people have not been married long. They are still in the honeymoon stage of marriage. Della will get tired of wearing old clothes and pinching pennies. Jim will get tired of his dead-end job. Then they could easily get tired of each other.