What qualities do Mercutio and Benvolio share?
Benvolio and Mercutio are both very loyal to Romeo and the Montagues. Although Benvolio is Romeo's cousin, both he and Mercutio are best friends and confidants to Romeo. Throughout Act I, they both worry about Romeo's melancholy regarding Rosaline. In Act II, they are concerned when Romeo goes off to profess his love to Rosaline.
They are both also involved in the feud between the Montagues and Capulets, even though Mercutio is not part of either family (he is in fact related to the Prince). Mercutio and Benvolio are both negatively affected by the feud and it even takes Mercutio's life in Act III.