At least at the beginning of the play, Juliet serves as Romeo's dramatic foil in being the sensible one, while Romeo is the emotionally driven one. However, as the play continues and the couple progresses deeper into their relationship, Juliet becomes emotionally driven as well.
We especially see Juliet being more rational and sensible than Romeo in the balcony scene found in Act 2, Scene 2. For example, Romeo begins this scene by speaking rapturously of Juliet's beauty and idolizing her by comparing her to heavenly beings, such as the "sun" (3), the "fairest stars in heaven" (15), and a "bright angel" (28). Juliet, on the other hand, opens the scene by laying out the problem of the family feud and rationalizing that it is really only their names that are standing in the way of their romance, and names mean nothing. We see her laying out the problem of the family feud when we see her ask herself, "O Romeo, Romeo! wherefore art thou Romeo?" (35). In asking why he must be Romeo, she is asking why he must be the son of a man her own father is fighting against. We see her rationalize that names are meaningless when we see her declare, "What's a Montague? it is nor hand, nor foot...nor any other part / Belonging to a man" (42-44).
We further see Juliet play the rational dramatic foil to Romeo's rash emotionalism when we see her argue that they are being to hasty in exchanging vows so suddenly, as we see in her lines,
Although I joy in thee,
I have no joy of this contract to-night.
It is too rash, too unadvis'd, too sudden. (122-124)
Thus, since we see Juliet remaining calm and rational while Romeo is impetuous and governed by his emotions, we can easily see Juliet as a dramatic foil to Romeo. However, Juliet ceases to be Romeo's dramatic foil when she, too, gives way to her emotions. She gives way to her emotions in letting Romeo convince her to marry him so suddenly, even though she thinks it is unwise. She further gives way to her emotions when she decides to commit suicide, first in order to escape marrying Paris and then in order to join Romeo in death.