It's worth remembering that A Christmas Carol isn't really a 'serious' book; it's more like the Victorian equivalent of a feel-good Hollywood Christmas blockbuster. Though the characters are complex in many ways, they're all too cliched to be seen as realistic. They're not real people, they're charicatures - like the picture to the left here.
All their good points and faults are exaggerated in a way that never happens with real people. But if you pretend that characters need to be realistic to be taken seriously, you're missing something important about fiction and literature in general.
Fred
Fred is another happy soul. He embodies the spirit that Dickens wanted to imbue us all with - a reminder that "Christmas is a good time; a kind, forgiving, charitable, pleasant time; the only time I know of, in the long calendar of the year, when men and women seem by one consent to open their shut-up hearts freely, and to think of people below them as if they really were fellow-passengers to the grave, and not another race of creatures bound on other journeys. And therefore, uncle, though it has never put a scrap of gold or silver in my pocket, I believe that it has done me good, and will do me good; and I say, God bless it!" That speech was what Dickens called his Carol Philosophy.