A dramatic foil is a character who is the complete opposite of another character.
A good example of this is in the play, The Tragedy of Romeo and Juliet.
Lady Capulet, Juliet's mother, is more concerned with the family, and their reputation rather than what Juliet wants. She uses Juliet as a tool. The nurse is more concerned with Juliet, and what she wants. Even if it goes against her mothers wishes. NURSE: "This afternoon, sir? Well, she shall be there" (II.IV.90-91)
Another example is Romeo and Tybalt. When Tybalt wants to fight Romeo, Romeo is persistent that he does not want to fight him because they are now family. Romeo is compassionate and has thick skin, while Tybalt is mean-spirited and easy to anger. "Tybalt, the reason that I have to love thee Doth much excuse the appertaining rage To such a greeting: villain am I none; Therefore farewell; I see thou know'st me not." (III.II.60-63)