When Traddles first appears as one of David's school friends at Salem House, we think he's just going to be a random one-off, the kid who draws skeletons whenever he's beaten to remind himself that these troubles will pass. David is so occupied with Steerforth that Traddles doesn't get too much of his narrative attention. At the same time, Traddles is so sweet and delightful that he's hard to ignore: he is the most frequent victim of Mr. Creakle's sadistic abuses, but he is also the kindest-tempered of all of the boys. When Steerforth drums Mr. Mell out of his teaching position because Mr. Mell's mother is in the nineteenth-century equivalent of a homeless shelter, Traddles is the only one who is willing to tell Steerforth that he's being a huge jerk.
When grown-up David runs into Traddles in London, we start to see that there are significant similarities between Traddles's life and David's own. Traddles is also an orphan. He was raised by an uncle who eventually decides he doesn't like Traddles much. The uncle dies without leaving Traddles anything in his will. Left without any money to continue his education, Traddles moves to London to become a law clerk. He works tirelessly to save up enough money to pass the bar and start his own law office. Through hard work and economy, Traddles slowly climbs up the legal ladder. Perhaps we could think of Traddles as David without either the factory life to mess him up or Miss Betsey Trotwood to save him.
Traddles's personal life also echoes David's in interesting ways: Traddles is in love with Sophy Crewler, a clergyman's daughter. Traddles knows that it will be years before he can afford to marry his beloved Sophy and set up a household of his own, but he waits patiently, dreaming always of the day when he can finally settle down as a married man. Traddles and Sophy are like the couple that David and Agnes would have been if David had not been foolish enough to fall for silly Dora first. And by the end of the novel, the large happy households of Traddles/Sophy and David/Agnes mirror one another.
Like David, Traddles must make his own way in the world through patience and hard work. Like David, Traddles winds up becoming entangled with the private lives of the Micawbers (a lucky accident, since they need a legal mind on hand to figure out what to do with Uriah Heep when that whole mess goes down). Like David, Traddles has the support of a faithful, good-hearted homemaker who gives him the happy home he deserves. And like David, Traddles has an affectionate heart: his sympathy for Mr. Mell, for the Crewlers (his in-laws), and for the Micawbers is what leads him to such professional and personal success in this novel. Traddles's life is like David's story of development as it would have been without Steerforth and Dora dragging David down.