Benvolio isn't the flashiest of characters in the play, and sometimes he gets a bit lost amongst the other rash and boisterous fellows.  And yet, he provides a strong base of reason and calm in play filled with hasty action and extremity of behaviour.

The opening scene of the play contrasts Benvolio and Tybalt nicely.  When both happen upon the street fight between the servants of the houses of Capulet (Tybalt's family) and Montague (Benvolio's family), their reactions are completely opposite:

Benvolio

Part you fools, put up your swords, you know not what you do.

Tybalt

What, art thou drawn among these heartless hinds?

Benvolio

I do but keep the peace, put up thy sword

Or manage it to part these men with me.

Tybalt

What, drawn, and talk of peace?  I hate the word

As I hate hell, all Montagues, and thee:

Have at thee, coward.

And this is the contrast between the two, in a nutshell.  Benvolio only has his sword drawn in order to stop the fighting and attempts to reason with Tybalt to calm down and work for peace -- to follow the Prince's rules.  Tybalt, on the other hand, insists that Benvolio's sword being drawn is a provocation to fighting, and when he can't incite Benvolio to strike first, he calls him a coward and lunges at him.

So, whereas Benvolio is calm, reasonable and most willing to keep the peace; Tybalt is hot-headed, somewhat irrational in his conclusions, and totally looking for some excuse to fight.  This set-up of their differences in Act I is carried through in the street fight of Act III, when it is Benvolio who tries to prevent Mercutio and Tybalt from a heated argument -- an argument that leads, ultimately to both of their deaths.

For more on these characters and Act I, scene i, please follow the links below.