Romeo and Benvolio have a friendly relationship. They are best friends and say to each other everything. Benvolio is a peaceful and calm character. Romeo, instead, is a good and loyal friend to Benvolio. They are cousins, Romeo and Benvolio, and they are both of the same family: Montague.
When Tybalt, the cousin of Juliet, a Capulet, provocates Benvolio, trying to make Benvolio fight, Benvolio tries to calm Tybalt instead of fughting with him. Benvolio always, in every situation, tries to bring peace in a firing soul.
For example, when Tybalt and Mercutio start fighting, Benvolio tries to stop them from killing each other. Then Romeo arrives and distracts Mercutio, so Tybalt takes advantage of its distraction and he kills Mercutio passing the sword between the stomach and the arm of Romeo. So Mercutio died with a wound in his stomach.
Then for revenge, Romeo, than, in a moment of anger runs after Tybalt and kills him.
This proofs how strong the relationship between Benvolio and Romeo is.