An example of a foil is Tybalt and Benvolio. The foil between them is there traits between each other. Benvolio does his best to keep peace and harmony, while Tybalt is always trying to pick a fights. This is because of the feud between the Montague and the Capulet's which Tybalt takes very seriously.
An example of this is when Romeo goes to Capulet's masked ball. Once Tybalt recognizes his voice he talks about wanting to fight Romeo for crashing the party
While Benvolio in the beggining scene tries his best to stop two servants from Montague and Capulet's from fighting.