Beatrice and Benedick
These two characters exemplify the expression "the course of true love never did run smooth" (which Lysander says in A Midsummer Night's Dream, 1.1.134). The two appear to have had an earlier relationship, but the details given by Shakespeare are few indeed*.
The displays of aggression toward one another and the obsession evident for the other in the early scenes of the play suggest that they feel more for each other than simple animosity. The process through which they are tricked to confessing their love for one another is truly delightful, as their strong wits and deep eloquence are overcome by relatively simplistic trickery that suggests that they are only too willing to believe what they hear. Nevertheless, they continue to quarrel throughout the play as they are, as Benedick says, "too wise to woo peaceably" (5.2.61).
When Benedick decides that Beatrice loves him, he decides to be "horribly in love with her" (2.3.231); he then does his best to court her according to the fashion of courtly love, though he says finally that he was not "born under a rhyming planet" (5.1.39).
Footnotes
-
The earlier relationship is suggested at one point by an exchange between Don Pedro and Beatrice:
Don Pedro: Come lady, come, you have lost the heart of SignorBenedick.
Beatrice: Indeed, my lord, he lent it me a while, and I gave him use for it, a double heart for his single one. Marry, once before he won it of me, with false dice. Therefore your grace may well say I have lost it.(2.1.240-245).