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.
But, being honest isn't always realistic. Sometimes there isn't a good choice for the President, just two bad ones. And the truth is, he doesn't always choose the highest moral route--he likes to think he would, but he doesn't. That fact keeps us on the edge of our seats, because while he talks about his high morals and beliefs, his choices don't always reflect that, so we aren't sure what he's going to do. It create tension.
Think about the complexity in that one character for a second. The President says one thing, but sometimes does the other. He wants to see himself as a good person, but sometimes he plays dirty. So not only is the President in disagreement with his brother, but his identity is in disagreement also.
But let's get back to the character foil. Imagine if the brother didn't constantly argue against the President's morals. What if they always agreed and were on the same page? Unless there is some strong humor in there or great character voice or a killer sense of camaraderie, it would be boring! And we wouldn't see how they foil one another. We wouldn't see how great of a guy the President is because we couldn't contrast him against his brother.
Sometimes when we write, we get stuck on making two characters be "best friends"--no arguments, lots of laughs and smiles. But realistically, even best friends get in arguments, or at least irritated with one another. Try using that to your advantage. How can their differences highlight one another's strengths? How can it bring each character into focus? The President in 24 loves his brother, but it's their arguments that help paint a clear picture of them.