Age
Elizabeth assesses him to be “about thirty,” making him a bit older than his cousin Fitzwilliam Darcy.
Family and Situation
Colonel Fitzwilliam is the younger son of the Earl of -------, meaning that he has at least one elder brother. This means that he’s also the nephew of Lady Catherine de Bourgh and first cousins with Anne de Bourgh, Fitzwilliam Darcy and Georgiana Darcy. We are later told explicitly that he and Darcy share guardianship of Georgiana.
Colonel Fitzwilliam is not the heir to a great fortune, something that he cites while explaining why he cannot marry where he chooses. Given his fairly high rank in the military, however, his family must be willing to support the purchase of a commission.
We do not have a tremendous amount of information about the relationship between Darcy and Col. Fitzwilliam. They seem to be close enough that they visit their aunt together. Colonel Fitzwilliam also says that he knows Charles Bingley, who he describes as a “great friend of Darcy’s”, suggesting that they have mingled socially to some extend. However, Fitzwilliam says he only knows the Bingley sisters “a little”, suggesting that either he has managed to avoid their company or is too polite to weigh in honestly about them with Elizabeth. We do know that Darcy and Fitzwilliam are candid enough with each other for Darcy to tell the story of extracting Bingley from his latest crush, though not so close that Darcy explicitly says Bingley was involved.
Appearance
From Elizabeth’s point of view, we learn that he is “not handsome, but in person and address most truly the gentleman.”
Character
Throughout his time at Kent, the Colonel impresses Elizabeth with his generally jovial manners and certainly does not seem to have the trouble talking to her or the company more generally that Darcy does. Elizabeth wonders if he may have some romantic interest in her (and he certainly does flirt a bit), but we are never given concrete evidence of this in the text.
