Colonel Fitzwilliam is the youngest son of an Earl. His father was the brother of both Lady Anne Darcy and Lady Catherine de Bourgh, making him the cousin of Anne de Bourgh and the Darcy siblings, Fitzwilliam and Georgiana.
He is not particularly handsome and lacks money, but makes up for these faults with his charm. Although Austen never properly gave him a true first name, many fanfictions refer to him as Richard. He is about 30 years old, making him very close to his cousin, Fitzwilliam Darcy's, age. This could explain why he is Georgiana’s co-guardian. He describes to Elizabeth that he must marry for money in order to support himself, which is why the two of them can never be more than friends.
Though he's commonly addressed as "Colonel", we learn nothing more about his military career. In fanfictions he's usually depicted as a professional soldier in some branch of the Regular Army, but he could just as easily be serving in the Royal Marines, in a militia regiment like Colonel Forster, or be no longer in active service but still using his military title, like Colonel Brandon in Sense and Sensibility.
Personality Characteristics: On the brighter side his person and address show him to be "most truly" a gentleman. He is also described as being well-bred and capable of talking pleasantly and smart enough to enjoy talking with Elizabeth.