In fiction, a foil is a character who contrasts with another character in order to highlight particular qualities of the other character. A foil usually either differs drastically or is extremely similar but with a key difference setting them apart.
Romeo and Juliet Foils
Mercutio is outgoing were as Romeo is sensitve, shy, and reserved. Two common foils in Romeo and Juliet are Mercutio and Romeo.
Is love a tender thing? it is too rough, Too rude, too boisterous, and it pricks like thorn.
When Romeo is interacting with Mercutio you easily notice Romeo's seriousness especially on the topic of love. In the Queen Mab speech the audience can sense that Mercutio is not serious about much.
"O, then, I see Queen mab hath been with you... she gallops night by night... Through lovers’ brains, and then they dream of love!