What personality traits make Mercutio and Benvolio round characters in Romeo and Juliet?

Benvolio:  Benvolio has similar traits to Mercutio in that he is greatly loyal to his family.  He takes Romeo in under his wing, tries to cheer him after Rosaline refuses him, and stands by his side in his quest to find something to make his cousin happy.  Another trait that makes Benvolio a round character can be seen after the deaths of Tybalt and Mercutio.  When Lady Capulet assures the Prince that twenty Montague men were responsible for the death of Tybalt, Benvolio presents a nearly accurate representation of the events that had passed.  Instead of being entirely honest, Mercutio makes it sound as if he had no ability to stop Romeo in his attack on Tybalt.  Truthfully, he stood by and watched it happen.  This shows us that he is not a liar, but he is self-preserving when the full truth would incriminate him as well.