Foil Characters often are polar opposites of those they are juxtaposed against. In Shakespeare's Hamlet, there are a number of 'foils'. Caludius is a foil for Hamlet in that one hates deception and the other (Claudius) is VERY good at it. Hamlet distains all of the political intrigue and Claudius thrives on it.
Ophelia and Gertrude are also foils for one another. Ophelia is a virtuous maiden and a good daughter. Gertrude is not virtuous (according to Hamlet) and dishonors her house.