<span>The purpose of a character foil is to offer a contrast to another character, usually the protagonist. This, thereby, helps deepen the foiled character's personality, since he has something to battle against, to essentially fight for his values and/or methods. Basically, it heightens the characterization of the other character. </span>
It would be about over a thousand words. An average for 3-4 years olds is a few thousand words.
Anyone hiking the Appalachian Trail should remember to bring his or her rain gear.