Jimmy is easygoing with a good sense of humor. Notice how even the warden of the prison seems to like Jimmy and Jimmy is comfortable enough to joke with the warden. We learn he has a good heart by what the warden says to him:
"You're not a bad fellow at heart. Stop cracking safes, and live straight.”
This information about Jimmy's personality also foreshadows what Jimmy will do at the end of the story.
Jimmy Valentine likes the good (material) things in life. Notice that when Jimmy is released from prison he is not interested in nature but goes to a restaurant for a nice meal and a good cigar:
"Disregarding the song of the birds, the waving green trees, and the smell of the flowers, Jimmy headed straight for a restaurant. There he tasted the first sweet joys of liberty in the shape of a broiled chicken and a bottle of white wine—followed by a cigar a grade better than the one the warden had given him."
We see that Jimmy has compassion because he "tossed a quarter into the hat of a blind man sitting by the door."
We get a sense that Jimmy is young, good-looking, and well-built by this physical description of his appearance:
"Jimmy, looking like an athletic young senior just home from college..." and that he is sophisticated and likes to dress well by the way the clerk at the hotel is impressed with the way Jimmy acts and the clerk studies/admires the way Jimmy tied his tie:
"The clerk was impressed by the clothes and manner of Jimmy. He [the clerk], himself, was something of a pattern of fashion to the thinly gilded youth of Elmore, but he now perceived his shortcomings. While trying to figure out Jimmy's manner of tying his four-in-hand he cordially gave information."