William Price is the good brother in this book. He is Fanny's best friend and he treats his sister very well. And, interestingly enough, William actually has a lot in common with Edmund for much of the novel. Both William and Edmund behave in very similar ways towards Fanny throughout, and even Edmund treats Fanny like a younger sister for most of the novel. We suppose, then, it's no coincidence that Fanny considers them the two people that she loves best in the entire world (2.36).
William has always been one of Fanny's biggest fans and protectors. He even tries to instruct his family on how to take care of Fanny since her health is bad:
His last thought on leaving home was for her. He stepped back again to the door to say, "Take care of Fanny, mother. She is tender, and not used to rough it like the rest of us. I charge you, take care of Fanny." (39.3)
Henry also joins this group of men who are overprotective with Fanny and worry about her health. Concern for Fanny's health seems to be a sign that someone genuinely cares for her (42.16).
At any rate, William functions as a sort of ideal in the book: he's an ideal brother, he sets the standard for caring about Fanny, and he is also just an ideal young man. Unlike most of the other gents in the novel, William works in a bold and "manly" profession, has adventures, and is also outgoing and moral. This is a combination hardly any of the other men in the novel manage to pull off effectively. William, we applaud you for being cool.