And the answer: Mercutio.
In an effort to dispel Romeo's beliefs about dreams, Mercutio tells him about Queen Mab, the mythical fairy Queen who makes dreamers foolish with her tricks.
Although he is not a Montague or Capulet, Mercutio acts as far more than just an interested party in Shakespeare's epic family feud. Mercutio is a secondary character — he appears in just four scenes — yet his character is known for stealing the show. Originally introduced as Romeo's best friend, Mercutio has been interpreted as Romeo's foil: a character who is meant to juxtapose another so that audiences can clearly see the qualities of the protagonist, or in this case, Romeo.
Romeo is an avid romantic, who quickly falls in love and believes that love will conquer all. Meanwhile, Mercutio is a skeptic, who mocks Romeo's love and predicts its doom upon realizing which family Juliet belongs to. In many ways, Mercutio is an anti-romantic to compliment Romeo's intense romantic tendencies.
Perhaps the most encapsulating (or should I say, en-Capulet-ing) moment of Mercutio's character arrives in Act I, Scene IV. As he tells the tale of Queen Mab, Mercutio demonstrates his cynical side as well as his vivid imagination. Unlike Romeo, Mercutio doesn't believe that dreams can be omens, and thus the dream world that he creates is just that: a world to exist entirely outside of reality. Instead, dreams exist to him as a manifestation of worldly, real anxieties. Ultimately, it is this speech that sets Mercutio apart from Romeo and indicates that the former man is not a trustworthy confidant. It is for this reason that Mercutio remains unaware of the intensity of the love between Romeo and Juliet, as well as their eventual marriage.
Learn more about Mercutio and other Shakespeare characters here.