Which character is Juliet's mother's foil?

A foil is a character that highlights the qualities of another character, often through direct contrast; by generating a secondary character with differing personality traits, an author can use a foil to display the difference between two people. The foil of Juliet's mother, Lady Capulet, is the Nurse. 

Whereas Lady Capulet behaves with a great deal of emotional distance toward her daughter, the Nurse is affectionate and invested in Juliet's wellbeing. When it is suggested to Juliet that she get married at the tender age of thirteen, Lady Capulet replies to the girl's protestations that she must, "think of marriage now; younger than you, / Here in Verona, ladies of esteem, / Are made already mothers..." Lady Capulet seems to have little interest in her daughter's happiness; rather, she emphasizes the importance of obedience as a woman and wife. The Nurse, on the other hand, is directly involved with Juliet's disobedience. She serves as a messenger between Romeo and Juliet and helps arrange their secret marriage and its subsequent consummation. Eventually the Nurse does try to convince Juliet to marry Paris, but only after facing the wrath of Lord Capulet; this suggestion seems more like an attempt to protect Juliet from her terrifying father than a betrayal of Juliet's already existing marriage.

Ultimately, the Nurse plays a nurturing, supportive, motherly presence in Juliet's life, which highlights just how absent, cold, and dismissive Lady Capulet can be.