Mercutio isn't a flat character. It's often jested that Shakespeare had to kill Mercutio because he would've stolen the show should he have lasted the whole way.
Romeo: "A gentleman, nurse, who loves to hear himself talk and will speak more in a minute than he will stand to in a month."
is simple and can be described in one or two sentences.
the two flats in b flat are e and b flat
His opinion doesn't change about anything up to his point of death.
A flat character reveals only one character, and a round character is a character you know more about