Who He Is
Benvolio is Montague's nephew and Romeo's cousin. He is the one who tries to separate the fights and the cousin to whom Romeo ends up confessing what he suffers over Rosaline. The play also uses him as a witness: he is the voice that tells the Prince what happened, and he tells it well.
Trajectory
In Scene 1 of Act I, he tries to part the servants and is challenged by Tybalt. Later, before Montague, he reports the brawl and says he saw Romeo in the woods at daybreak. Alone with his cousin, he hears that Romeo loves a woman who has sworn chastity, and he advises him to forget her.
In Scene 2 of Act I, he proposes the cure: go to the Capulets' ball, where Rosaline will be, and compare her face with those of other girls. It is because of him that Romeo goes to the party where he meets Juliet.
In Scene 4 of Act I, he goes along with Romeo and Mercutio in masks, and after Romeo interrupts the speech about Queen Mab, he says that the wind Mercutio talks of blows them away from themselves and that supper is over.
In Scene 1 of Act II and Scene 4 of Act II, he looks for Romeo with Mercutio, finds out that he did not sleep at home, and knows of Tybalt's challenge, but says Romeo will answer the sender of the letter.
In Scene 1 of Act III, he wants to escape the fight on a hot day, and Mercutio describes him as a brawler. When the wounded Mercutio asks him to help him into a house, Benvolio returns to tell Romeo that he has died, and after Romeo kills Tybalt, he tells him to flee. He tells everything to the Prince, and Lady Capulet says that his partiality makes him false, being a kinsman of Montague. The Prince banishes Romeo instead of condemning him to death. After this scene, he does not return to the play.
Themes Embodied
Benvolio is an attempt at peace without power. His first words in the street are to separate, and those in Scene 1 of Act III are to leave before the fight; in neither case does the violence give way.
Benvolio is the one who sees earliest what will come: he tries to lift Romeo out of his sadness, tries to avoid the fight and afterward tells the Prince, accurately, what happened. In each case the play denies him effect: Romeo does not forget, Mercutio does not back down, and the exact account is received by Lady Capulet as a kinsman's partiality.