In Act I Scene 4 of Romeo and Juliet, Mercutio describes a character in great detail who never appears in the play. Who is it?

Queen Mab may not appear in the play as a character, but
she is certainly prevalent in the play.

Mercutio states
that lawyers will dream of lawyering, (paraphrase) butchers butchering, etc. "Dreamers
often lie," he says to Romeo, who responds, "In bed as they dream things true." Mercutio
is wrong, however, because Romeo's premonition of some occurence at the Capulet ball
(which will cost him his life) turns out to be
true.

Mercutio's humorous portrayal of Mab, and his
deflection of a serious concern of Romeo into a comedic situation, is synchronous with
his character. Mercutio often doesn't understand the full nature of a situation before
he becomes involved, just like his death scene.