How does Juliet's character help illuminate the work as a whole?

Juliet symbolizes everything having to do with light. For Romeo at the beginning of the play, he is in the dark clutches of loneliness and depression for having recently lost his first love Rosaline. At Lord Capulet's party he sees Juliet for the first time and says, "O, she doth teach the torches to burn bright!" (I.v.44). Juliet, like a torch, leads Romeo out of the darkness into the light of love. Then, it is the light in her window by the darkness of night that Romeo is guided through the estate's gardens. To Romeo, this is not just a light, but the sun, which symbolizes the most powerful light and energy that the world knows.

"But, soft! What light through yonder window breaks?

It is the east, and Juliet is the sun" (II.ii.2-3).

Then, Romeo compares himself to the moon and asks her for help out of the darkness of his grief, as follows:

"Arise, fair sun, and kill the envious moon,

Who is already sick and pale with grief" (II.ii.4-5).

Romeo continues to compare images that are filled with light to Juliet, such as, "The brightness of her cheek would shame those stars" (II.ii.19).

For other characters, and the play as a whole, everything happy depends on Juliet. For example, Friar Lawrence believes that the marriage between Romeo and Juliet could help to end the conflict between the two families. Paris wants to brighten his life by marrying her. Her father wishes to stop her tears, as well as his family's grief over Tybalt's death, by marrying her to Paris. All of these other characters hope to find a brighter life through Juliet marrying someone.

Juliet, herself, also brings brightness and hope to the play because of her youthful and happy spirit. She is the one who makes Romeo an honest man by proposing marriage first. This shows that she is pure and pious and wants to do what is right before God, even though she doesn't want to marry her father's choice for a husband. She also brings a youthful hope for a better world as her love for Romeo also shines through troubled times:

"Come, night; come, Romeo; come, thou day in night;

For thou wilt lie upon the wings of night.

Whiter than new snow on a raven's back.

Come, gentle night, come, loving, black-brow'd night,

Give me my Romeo; and, when he shall die,

Take him and cut him out in little stars,

And he will make the face of heaven so fine

That all the world will be in love with night

And pay no worship to the garish sun" (III.ii.17-25).

The struggle between darkness and light conflict and battle throughout the play as Juliet represents day and Romeo represents night; and the light of the stars will forever claim the "star-cross'd lovers" just as the Chorus announces and proclaims in the Prologue.