In Shakespeare's "Romeo and Juliet," the nurse and the friar are stock characters, and, as such, are stereotypical characters who are usually not developed. As such minor characters, they serve a single function and do not usually contribute to the major theme of the literary work. Therefore, they are neither protagonists or antagonists and are not involved in the denouement of the plot, either.
The primary purpose of the Nurse is to provide comic relief. For example, in Act III, she appears with excessive clothing, billowing like a sail that must be controlled by her servant, Peter. Mercutio jokes about her and the others laugh. This comedic relief comes before the tragic death of Mercutio and the misfortune of Romeo who is banished from Verona.
The friar, on the other hand, does serve more of a purpose in the play. For, it is his interference and blunders which move the play to its tragic end. As a stock character, however, Shakespeare employs his Friar to represent the corruption of the Catholic Church, whose priests went outside their vows to delve into potions (close to witchcraft) and become too involved in the secular life from which they have removed themselves in their vows. Thus, Friar Lawrence sins in his deceptive acts of giving Juliet the potion and his close involvement with Juliet and Romeo. Such acts are outside those dictated by the Church. When confronted at the tomb, he shows no fortitude or honesty; instead he runs away: "I can no longer stay!"