Hugh is Sylvie's husband and Ursula and her siblings' father. He's a hard-working husband, he loves his children, and they're all sad when he dies. Maybe because he's the least dramatic of everyone, we hear the least about him.
The worst we can say about him is that he's kind of oblivious: "Now you will never forget the anniversary of our marriage" (4.22), he tells Sylvie when they wed on her birthday—instead of, say, happy birthday, my love. Maybe this is why she's cheating on him… not that he realizes that either.
He's a doting father, although his daughters—like us—don't really know much about him. When Nancy Shawcross reads Robert Louis Stevenson at his funeral, she "surprised all the women of his family who didn't know that Hugh had any inclination toward poetry" (25.240). Why didn't they know? Maybe because they never asked him.