In Pride & Prejudice, what is Colonel Fitzwilliam's function?
He is basically a messenger and a positive informant for Elizabeth. Significantly, Elizabeth trusts and respects Fitzwilliam. While Mr. Bingley could certainly provide much of Darcy's background information to Elizabeth and clear up many of her misconceptions of Mr. Darcy, Elizabeth does not truly respect Charles Bingley. For one, he appears to have jilted her sister, and Austen portrays Elizabeth as being slightly disdainful toward Bingley. Fitzwilliam, on the other hand, is kind and serious-minded. He is related to Darcy; so he has a longer past with him and Wickham. Thus, Elizabeth believes him when he speaks well of Darcy and when he makes comments about Wickham's true character. He opens Elizabeth's eyes to a perspective she had not thought of before.
Likewise, because Fitzwilliam isrelated to the Lady Catherine de Bourgh, he caninform Elizabeth of the complicated relationship between Darcy, his cousin, and Lady Catherine.
Overall, he is a trusted confidant and champion for Darcy. He is similar to Slim from Of Mice and Men.