Lord ---- is an earl surnamed Fitzwilliam, who is the father of at least two children (his heir and Colonel Fitzwilliam), brother of Lady Catherine de Bourgh and the late Lady Anne Darcy, and uncle to Anne de Bourgh, Fitzwilliam Darcy, and Georgiana Darcy. In Pride and Prejudice, his title is omitted by a dash and he is referred to as "Lord ----" or merely "an earl." Jane Austen presumably intended her readers to make a connection to the real Earls Fitzwilliam.
While he never appears in the novel, his existence serves to show how well-connected and aristocratic Darcy is, and because he did not provide more generously for his younger son, Colonel Fitzwilliam is unable to pursue Elizabeth Bennet. His reaction to Darcy and Elizabeth's engagement is unknown, but he was probably less angry than Lady Catherine, if only because he did not plan for Darcy to marry his own daughter.