Scrooge's nephew, the son of his beloved but now dead sister, is his only living relative, and also the only person who wants to pull him out of isolation and back into the world.
Let's be honest—Fred doesn't have much of a personality. Sorry Fred, you know it's true. He's not really a three dimensional character who lives and breathes and does things that are supposed to simulate the kinds of things that real people in the real world do.
And he isn't really supposed to be. Instead, his purpose in the story is to be Scrooge's bridge back into human relations and to show that when Scrooge isolates himself in his horrible little apartment on Christmas, he is actually making a choice to be alone rather than forced to be that way by circumstance.
Because we know that Scrooge has an alternative—he could go to Fred's party, get drunk, boogie, and be a normal person—we realize just how far his bitterness and miserliness have taken him out of the world.