In Jane Austen's Pride and Prejudice, Colonel Fitzwilliam is Mr. Darcy's cousin and second nephew of Lady Catherine de Bourgh. We first meet him at Mr. Collins' parsonage at Rosings. Elizabeth and Maria travel to Hertfordshire to visit with Charlotte. Shortly after they arrive, Mr. Collins observes that Mr. Darcy and Colonel Fitzwilliam have arrived to visit with their aunt. He goes to Rosings to pay his respects and brings both men back to the parsonage to call upon the women (Ch. 8, Vol. 2).
Elizabeth likes Fitzwilliam very much and they have many conversations together during her stay at the parsonage. She even encounters him several times while walking the grounds of Rosings. During one encounter, she learns from Fitzwilliam that Darcy persuaded Bingley not to marry Jane (Ch. 10, Vol. 2).
We also learn in Darcy's letter to Elizabeth that Fitzwilliam is co-guardian of Miss Darcy and knows all about how Wickham tried to seduce Miss Darcy.