Fred Vincy is probably the least important of the seven major characters, simply because he's the least complicated. He's sincere, blunt, and honest, smart enough to do a good job at most things, although foolish enough to make a mess of things occasionally. He loves his mother and he loves Mary Garth.
Fred might seem like a fool to most people, but he's the favorite of Mrs. Vincy: "The mother's eyes are not always deceived in their partiality: she at least can best judge who is the tender, filial-hearted child. And Fred was certainly very fond of his mother" (2.14.89). Rosamond might be beautiful and proper, but her affection for her mother is part of her act. And mothers, the narrator informs us, can always tell the difference. Fred's sincerity and affection are appreciated by his mother, at least.
He seems like a fool to Mary Garth, too, but she loves him anyway. But why does he love her? His family is well off, and he's personally very good-looking. And yet Fred "was thoroughly in love, and with a plain girl, who had no money!" (2.14.86). So what's the attraction for him?
Well, the easy answer is that he has always loved Mary. They were childhood sweethearts. And asking a young man as honest, sincere, and affectionate as Fred Vincy to stop loving his childhood sweetheart would be like asking him to stop breathing. Maybe it's more complicated than that, though – it's possible that Fred realizes that he needs someone with some common sense to keep him from getting into trouble. Mary looks out for him and keeps him from making a fool of himself. After all, she's the one who saved him from becoming a clergyman – a career that would have made him very respectably rich, but which he would have absolutely hated.