Tolliver works for a government agency, managing what we would today call covert operations. When Jude first met him, when she was writing Revealed in Mist, all she knew about him was that Prue Virtue, the heroine, worked for him as a spy.
Over the course of that book, readers discover that he had offered David Wakefield employment back when David’s father, the Duke of Haverford, was systematically blocking any attempt David made at independence. And then that the Marquis of Aldridge, David’s half-brother, called him Uncle Tolly.
His full name, which he eventually disclosed to David, gives the clue. Thomas Oliver Fitzgrenford is Haverford’s younger half-brother. The duke and the illegitimate son have been estranged since Jonathan was a baby but, as we find out in Paradise Lost, the duchess’s memoirs, Tolly has continued to work with the duchess to help her protect Haverford’s children and grandchildren, both base-born and legitimate.
He will continue to appear in Jude’s books in either his government role or that private family role.