How does Shakespeare portray the Nurse in Romeo and Juliet?

The Nurse is set up as a foil to Juliet. A foil is a character who provides a contrast to another character. The Nurse is more light-hearted than Juliet. She doesn't look at love as romantically as the girl and often refers to it simply as a physical act. She obviously loves Juliet very much or else she would not have provided the support that allowed Juliet to be with Romeo. She is also more pragmatic and when Romeo is banished she urges Juliet to marry Count Paris in order to please Lord Capulet.

Her light-hearted and whimsical nature are on display when she is introduced in Act I, Scene 3. When asked how old Juliet is, she launches into a bawdy story about how her husband made a rude comment about Juliet when she was a little girl. She goes on and on, much to the dismay of Lady Capulet who seems uptight and nervous around her daughter. The Nurse oozes love for Juliet. She says, "Thou wast the prettiest babe that e’er I nursed." In this scene and in Act II, she often provides comic relief in a story which is moving toward tragedy.

As indicated by her story in Act I, Scene 3, the Nurse looks at love in a lustier, more physical way. In that scene she is simply concerned that Paris is good looking. That he may provide the spiritual link Juliet is looking for does not concern her as illustrated by her comment that men make women bigger by getting them pregnant: "No less? Nay, bigger. Women grow by men."

After she meets Romeo she can only talk about his good looks. She admits that he is not the most courteous man she's ever met but that doesn't matter because he is physically attractive. In Act II, Scene 5 she says,

Though his face be better than any man’s, yet his leg
excels all men’s, and for a hand and a foot and a
body, though they be not to be talked on, yet they
are past compare.

A little later in the scene she makes another sexual reference about Juliet's honeymoon night. The Nurse acts as a go between for Romeo and Juliet and is responsible for getting a rope ladder so Romeo can crawl into Juliet's room. She says,

Hie you to church. I must another way,
To fetch a ladder by the which your love
Must climb a bird’s nest soon when it is dark.
I am the drudge and toil in your delight,
But you shall bear the burden soon at night.

The Nurse does not understand the spiritual nature of Juliet's love. Juliet believes Romeo is her soulmate, not simply a good looking man who will provide her with physical pleasure (though Juliet is not without anticipation of this). Because the Nurse fails to recognize, or simply cannot comprehend, Juliet's intense feelings for Romeo it is easy for her to advise Juliet to forget him and do as Lord Capulet wishes. In Act III, Scene 5 she tells Juliet to marry Paris,

Then, since the case so stands as now it doth,
I think it best you married with the County.
O, he’s a lovely gentleman!

That the Nurse is kind, loving and trying to do what is best for Juliet cannot be denied. She goes out of her way for the girl and probably risks her job and maybe even her life by keeping Juliet's secret. In the last scene where the Nurse appears, Shakespeare reinforces the idea that the Nurse truly loved Juliet as she cries and carries on after finding Juliet supposedly dead at the end of Act IV:

O woe, O woeful, woeful, woeful day!
Most lamentable day, most woeful day
That ever, ever I did yet behold!
O day, O day, O day, O hateful day!
Never was seen so black a day as this!
O woeful day, O woeful day!