if two main characters are opposite, they are called "foils" of each other (not paradoxes. when you're teacher said that romeo and mercutio being opposite is a paradox, she was wrong). the purpose of a foil is to contrast each other and over-emphasize a certain trait of the person they're foiling, whereas a paradox is meant to reveal a truth about something, usually in an absurd and akward way. although pretty late, a few good examples of paradoxes would be:
1.5.134-135 Juliet: If he be married, My grave is like to be my wedding bed.
3.2.130-136 Juliet: But I, a maid, die maiden-widowed...I'll to my wedding-bed, and death, not Romeo, take my maidenhead!
3.5.199-200 Juliet: Or if you do not, make the bridal bed in that dim monument where Tybalt lies"