Rawdon's older brother Pitt seems at first to be an effeminate and pedantic weakling. However, his powers as a diplomat and master strategist eventually get him most of the material prizes the novel has to offer: a seat in Parliament, an introduction at court, the Crawley fortune, the Queen's Crawley estate, and a baronetcy.
Pitt starts out as kind of a non-character. When Becky first meets him, he is prissy, sickly, and horribly in love with his questionable intellect. But then something changes. He gains dimensions and turns from a caricature into a much more realistic character. We find out about his stalled diplomatic career and then see him and his people skills in action as he positions himself to be Miss Crawley's heir. We discover that he is an excellent steward of the Queen's Crawley estate, and that instead of simply hoarding his money, he has instituted a full-scale restoration of the grounds as well as the abandoned country and city mansions. Finally, although he remains a cold fish, we get a glimpse of his sense of morality and loyalty as he takes in and raises his nephew for the sake of his brother.
This development from one-note persona to three-dimensional personality is probably a result of the way Thackeray's idea of Vanity Fair shifted during writing (see Shmoop's "In a Nutshell" for all the gory details). But it's a technique that Thackeray uses often with secondary or even tertiary characters in his novels. Can you think of other characters who get the same treatment – who start out as types and become individuals?