Romeo is the son of Lord and Lady Montague, sworn enemies of Lord and Lady Capulet. His emotions run from the lowest of lows to the highest of highs.
His parents worry about his sadness. At the end of the brawl in Act I, sc i, the Montagues cannot find their son Romeo and ask Benvolio if he has seen him. Benvolio mentions seeing Romeo before dawn walking "underneath the grove of sycamore" but Romeo ran away from him.
His father has become weary of Romeo wandering around crying during the night, only to come home during the day:
"Away from light steals home my heavy son,
and private in his chamber pens himself,
shuts up his windows, locks fair daylight out,
And makes himself an artificial night, (Act I, sc i, lines 127-130)."
Romeo is despondent over his unrequited love for Rosaline, who has taken a vow of chastity; in just a few hours he will be over the moon in love with Juliet. Romeo is also secretive, spontaneous and connyving when it comes to getting what he wants.
Mercutio is a friend of Romeo; he is the joker of the group; he is incredibly witty and has perfected the double entendre (usually lewd or sexual in nature) typical of teenage boys. He is the prankster amongst the group and the friend the rest of the Montague boys look to as a source of entertainment. Mercutio is also loyal to Romeo and unabashedly unafraid of standing up for his friend.
Mercutio - wild, impulsive, party-er type
Romeo - impulsive, immature, childish, infatuated, "softie"