I've been watching 24, and in it, the President of the United States has a very obvious character foil: his brother. I'm pretty much convinced that the brother's main purpose in the story is to be a foil. See, the President has really high morals. That's great. But the audience doesn't know that unless the President is constantly faced with immoral opportunities.
That's where his brother comes in. While they love and care about one another, the brother is often trying to get the President to play dirty. So we get to see the President face immoral choices again and again and listen to him talk about doing what's right again and again. Without his brother, we wouldn't see that.