Foil
Character Role Analysis
Tristram Foxe and Derek Foxe
As brothers who look a lot alike and are probably pretty close in age, Derek and Tristram definitely form a foil. And, just in case the significance of their pairing wasn't entirely clear, The Wanting Seed gives us a second Derek and Tristram (the bouncing baby boys) just to hammer home the point.
If you think about Tristram and Derek as flip sides of the same coin, it's easy to see that each of them takes a very different view of the world. Derek is the kind of guy who will turn any situation to his own advantage, even if it means pretending to be someone he's not. Tristram, on the other hand, couldn't hide his true feelings and opinions if he tried. Whether you think of that as a weakness or a virtue is up to you. Is Derek a smooth operator, or a hypocrite? Is Tristram an Everyman, or a patsy?