Our central insight into Jim and Della's characters comes from the action at the heart of the story: their mutual sacrifice of their most treasured possessions to buy each other gifts. Can't have a more clear proof than that of how good they are and how much they love each other. The actions also speak for themselves. We don't get into Della's head enough to see her make up her mind to sell her precious hair; we just watch her go to Madame Sofronie's and do it. Della's other action – fixing up of her hair, preparation of coffee and the pork chops – further reveal her love for Jim.
We barely get into Jim's head at all, and only have limited knowledge about what Della's thinking, but whenever the narrator tells us about Della's thoughts, he makes it apparent how much she loves Jim. Her thoughts are almost always about him. She's absolutely fixated on buying him a present, we know from what she's thinking, and she thinks it needs to be something worthy of Jim. We are told how excited she feels once she's found the right gift. We also know from Della's thoughts how concerned she is that Jim still find her beautiful after she's sacrificed her hair.
We don't get too many details about Jim and Della's clothing, but it's enough to establish them as humble folks. Della wears an old brown hat and old brown jacket, Jim a worn-out overcoat and no gloves, in spite of the cold. Della's attire contrasts with her excited movements, her "sparkling" eyes, and her evident beauty. That inner/outer contrast is at work here too.
The various features of Della and Jim's dilapidated flat – its broken mailbox and doorbell, cheap mirror, and worn-out rug – all make their poverty quite apparent. Their living space performs much the same function as their clothing: establishes them as humble people, and helps illustrate that it's not money or external values which matter to them. The only thing that matters is their love for each other.
We get a hint at Della's inner warmth and energy by the repeated mention of the "brilliant sparkle" in her eyes (11). Jim looks "serious" (25), and in fact, seems to be quite serious.