What is the relationship between Romeo and Mercutio from Shakespeare's Romeo and Juliet?
Romeo and Mercutio are just good friends, and that is the extent of their relationship. Romeo and Benvolio are cousins, and Mercutio is related to the Prince, as the Prince states after the confrontation when Tybalt slays Mercutio and Romeo then slays Tybalt.
When Tybalt approaches Mercutio, looking to fight Romeo, he says, "Mercutio, thou consort’st with Romeo." In other words, Tybalt knows Mercutio as someone who is friends with Romeo, not someone who is related to him in some other way. Then, after Tybalt has killed Romeo, Romeo...
moves to attack him, insisting that one of them must join Mercutio on his way to heaven. Again, Tybalt refers to Mercutio as Romeo's friend, saying, "Thou, wretched boy, that didst consort him here / Shalt with him hence." Again, Mercutio is identified as someone with whom Romeo consorts, or hangs out with.
When the Prince comes to hear what has happened, he learns that Tybalt slew Mercutio, and then Romeo slew Tybalt. When he addresses the two families, he says, "I have an interest in your hearts' proceeding. / My blood for your rude brawls doth lie a-bleeding." He means that blood of his family has been spilled as a part of the long-standing Capulet-Montague feud; thus, we know that he is related to Mercutio, but Romeo is not.