Benvolio and Tybalt are foil characters. They are both male, around the same age. Benvolio is Romeo's cousin, Tybalt is Juliet's cousin. When we first see them in Act I, scene i they have come upon a street scene with the servants fighting. Their respective reactions to the fight help the viewer define who these characters really are and predict how they will react to future events.
When Benvolio sees the fight, he attempts to break it up. He seeks peace either because he is a peaceful person or he fears the consequences of the fight. When Tybalt appears Benvolio asks him to help bring peace. Tybalt answers, "Peace? I hate the word. As I hate hell, all Montagues, and thee." Instead of breaking up the fight, he adds to its violence which leads to the proclamation later by the Prince.
In this initial scene Shakespeare sets the mood of the play. Tension and violence will be present throughout. Benvolio will try to lessen it; Tybalt will add to it. This part of the scene is quickly followed by Romeo and Benvolio's discussion of love, so the whole love/hate thing is established from the very beginning of the play.