In Act II, Mercutio and the Nurse play important roles. How are they character foils to Romeo and Juliet?
Both Mercutio and the Nurse make ideal foils for Romeo and Juliet in Shakespeare's The Tragedy of Romeo and Juliet, as their positions and their personalities highlight the defining traits of both Romeo and Juliet. Neither Mercutio nor the Nurse are blood relatives of the characters, so their connections to Romeo and Juliet are close, but distant enough to enhance the contrasting effect of the foil. Mercutio is a friend, not a sibling nor a cousin, so he can be objective to Romeo, while the Nurse can also speak somewhat openly with Juliet as she is employed by the family and not a family member.
Mercutio is a good friend of Romeo's, and he is as skeptical as Romeo is dreamy. Mercutio is a witty and practical-minded friend to Romeo, pointing out Romeo's excessive and needless emotion when he lets his romantic fantasies get the better of him. An example of this pragmatism can be found in Mercutio's reaction to Romeo's lamentations over Rosaline, whom Mercutio dismisses as false.
Juliet and the Nurse enjoy a close relationship, but as much as the Nurse cares for Juliet, she does not take Juliet's side just because of this. For example, the Nurse supports Juliet's marriage to Paris as much as Juliet opposes it, and believes that Paris is a good and worthy match for Juliet. The Nurse compares Paris to Romeo, and she finds Paris to be a much better fit for Juliet. Of course, the Nurse ends up being right about Romeo as he directly causes Juliet's early death.
In Romeo and Juliet, William Shakespeare frequently used characters as foils, which simply means that those characters were used to contrast other characters. In Act II of the play, Mercutio acts as a foil to Romeo, while the Nurse is a foil to Juliet.
In Scene V of Act II, Nurse arrives back at the Capulet home after having met with Romeo. Juliet, who is desperate for whatever news Nurse has for her, is impatient and demanding; she cares about nothing more than her marriage and has no interest in, or patience for, other topics. Nurse, however, is worn out from her excursion and complains that her bones ache, she is out of breath, and her head and back hurt. Although Nurse does have the news Juliet wants so badly, Nurse prefers to ramble and "beat around the bush" before she delivers Romeo's message.
Mercutio acts as Romeo's foil in Scene IV. Mercutio tries to impress on Romeo the belief that it is much more fun to enjoy life that be depressed about love all of the time. Mercutio does not realize that Romeo is no longer heartbroken over Rosaline.