What is a foil? It's a character that has several qualities opposite another (usually the main character), designed to enhance and boost the opposite character's traits. Say you have a brave character that you really want to appear larger than life. You make his sidekick or best friend (or somebody else that he hangs out with a lot) a complete wimp. It makes your brave character look even braver by comparison.
Whether I do it intentionally or not, I have used foils in the past and will likely to continue to use them. There are some times where it just naturally falls into place. I can think of two sets of characters (both from tSotD) right off the top of my head.
Pair one: Valerian and Astra
These are my main characters. Astra is a very angry, intense, fiery, stomp-all-over-things person. Contrasting that, Valerian is level-headed, cautious, guarded, and though he has a temper, he rarely shows it. They foil each other and, to me, it enhances the qualities of both by comparison.
Par two: Adolphus and Bran
Adolphus is the leader of the resistance movement. Bran is second in charge. Adolphus is very structure-oriented, strict, take-charge, temperamental person. Bran is in favor of structure, but also believes in spontaneity and relaxed rules in some cases. He is more laid-back and far more objective than Adolphus. Again, they foil each other.
What about you guys? Do you use foils?
As the Plot Thickens / High on Life / The DelusionalI shall never grow up. Make believe is too much fun.