What song represents each character in Romeo and Juliet?

I'm not sure what your question is referring to so I will offer a couple of alternatives. If you're asking about modern songs that could be applied to Shakespeare's Romeo and Juliet I've provided a couple of links below that may be helpful.

You might also be referring to the beginning of Act III, Scene 5. It is the morning after the two have consummated their marriage and Juliet does not want the night to end. She says she hears a nightingale singing outside her window. The bird, of course, is the symbol of the night. She tells Romeo,

Wilt thou be gone? It is not yet near day.
It was the nightingale, and not the lark,
That pierced the fearful hollow of thine ear.
Nightly she sings on yond pomegranate tree.
Believe me, love, it was the nightingale.

In an interesting twist on the theme of dark and light in the play, the two lovers are safe under the dark of night. The light of day, however, will take Romeo away and expose him to "fickle fortune." Romeo, in a juxtaposition on the theme, says,

More light and light, more dark and dark our woes.

Romeo, knowing he must leave Verona or be apprehended by the Prince, realizes day is breaking and says it is the lark singing:

It was the lark, the herald of the morn,
No nightingale. Look, love, what envious streaks
Do lace the severing clouds in yonder east.
Night’s candles are burnt out, and jocund day
Stands tiptoe on the misty mountain-tops.
I must be gone and live, or stay and die.

This is the only reference to songs in the play but they make for a truly beautiful exchange between the "star-crossed" lovers. In Franco Zeffirelli's 1968 movie, a renaissance troubadour sings a song titled "What is a Youth" which speaks of young love and is appropriate to the play. In Baz Luhrmann's 1996 version he uses a quite haunting song, "Everybody's Free" sung beautifully by Quindon Tarver. The songs title is ironic given the plot of the play and how Romeo and Juliet are at the mercy of fate.

If your question refers to popular songs, I have two favorites which reference the characters directly. One is an old song by Dire Straits called "Romeo and Juliet." The song brings up images of a lovestruck Romeo and the stars above, both quite in line with the imagery in the play. Another is We the Kings' song "Check Yes Juliet" which opens with images of a young man outside a window waiting for a girl, much as Romeo waits for Juliet at her balcony. The song also talks about two lovers running away from those who would tear them apart.