In Great Expectations by Charles Dickens, several characters contribute to the plot and themes of the novel. One of these many themes is that wealth can become a tool of self-destruction. Many readers of the novel regard Trabb's Boy and Clara as minor characters. They are indeed minor in comparison to the rest of the book, although each of them plays a major role in developing the theme of wealth and its detrimental aftershocks for Pip and Herbert Pocket.
Pip takes his goal to become a "gentleman" quite seriously, and to some degree takes this too far, thus becoming a snob rather than using his status as a compliment to himself and others. This is extremely clear when Pip runs into Trabb's Boy in town. As Trabb's Boy sees Pip strutting around London, he begins to mock Pip for his pompous behavior, mimicking his movements and general persona. This upsets Pip very much, so much so that Pip sends correspondence to Mr. Trabb saying he will refuse to patronize his business from now on. This scene with Trabb's Boy is important because it shows Pip's detachment. Through gaining wealth and stature, or what Pip perceives as stature, Pip forgets who he is and where he has come from. Pip alienates his true self and the ones that love and/or care for him such as Joe, Biddy, and the people back home like Mr. Trabb.
As for Clara's importance to the similar theme of the corrupting nature of wealth, she also plays a major role. Herbert Pocket feels the need to secretly be engaged to Clara, because she fails to live up to Mrs. Pocket's expectations of wealth and social stature.
Again we can see that these seemingly minor characters serve to illustrate how far off the mark Pip and Herbert are in their quest to be gentlemen. Clara must be hidden away from Herbert's family only because of financial and snobbish aims. Clara represents purity of character, and that wealth does not matter as much as love and loyalty.