The characters of Ebenezer Scrooge and his nephew, Fred, are juxtaposed (described closely together in the text), which brings out their contrast even more fully. Ebenezer is described as
a squeezing, wrenching, grasping, scraping, clutching, covetous, old sinner! Hard and sharp as flint, from which no steel had ever struck out generous fire; secret, and self-contained, and solitary as an oyster. The cold within him froze his old features, nipped his pointed nose, shrivelled his cheek, stiffened his gait; made his eyes red, his thin lips blue; and spoke out shrewdly in his grating voice . . . He carried his own low temperature always about with him.
He is described as cold in every way: in terms of his temperature, his appearance, and his emotions. Put bluntly, he is unfeeling and totally lacks compassion. He is greedy, unkind, and gruff. His nephew, Fred, on the other hand,
had so heated himself with rapid walking in the fog and frost, this nephew of Scrooge’s, that he was all in a glow; his face was ruddy and handsome; his eyes sparkled, and his breath smoked again.
While Scrooge is everything cold, Fred is warm, both literally and figuratively. He's physically heated himself up with quick walking, and he is all aglow, with a ruddy face, sparkling eyes, and smoky breath. Everything about his description makes him sound warm, and he is warm in terms of his personality, too. He champions Christmas, helping others, and finding joy, while Scrooge would rather be left alone to himself and his business.
The juxtaposition of these two characters allows us to really see the full measure of their contrast. In addition, seeing Fred's Christmas dinner with the Ghost of Christmas Present helps to "thaw out" Scrooge, making him less cold emotionally.