Rosaline is the Princess's wingman (just like Berowne is the King's wingman). We probably wouldn't fight you if you were to argue that Rosaline is one of the play's protagonists. Like Berowne, she's kind of a truth-teller, and she's also smart, funny, and has a dark side.
In fact many scholars believe Rosaline to be a reflection of the "dark lady" addressed by the speaker of Shakespeare's Sonnets 127 to 152, and this "dark lady" is thought to have been Shakespeare's mistress (check out the "In a Nutshell" section of Sonnet 18 for more information on the Sonnets). If we think of Berowne as a reflection of Shakespeare himself, and if we think of Rosaline as a version of Shakespeare's mistress, then we have an interesting relationship on our hands. Even though the King of Navarre and the Princess of France are the rulers and tone-setters, we can't help but focus more of our attention on the romance that develops between Berowne and Rosaline. In an attempt to get to know Rosaline as a character, let's consider several key aspects of her personality.
In the first place, Rosaline has the rare linguistic ability to give Berowne a run for his money in the arena of witty banter. In fact, she's as enthusiastic and talented at wordplay as any of the men. Take a look at this interaction between with Berowne as the ladies tell the gentlemen about a band of "Muscovites" who have just passed through (knowing full well that the "Muscovites" we just the gentlemen in disguise):
BEROWNE: I am a fool, and full of poverty.
ROSALINE: But that you take what doth to you belong,
It were a fault to snatch words from my tongue.
BEROWNE: O, I am yours, and all that I possess.
ROSALINE: All the fool mine?
Notice how she drives home the point that Berowne is a fool, even after he affectionately professes his love for her. Words and wordplay are important to Rosaline and Berowne's relationship, and she can bend his words and expose his weaknesses with a great deal of skill. We feel she's distrustful of Berowne and of love, and we also know that the two have a somewhat mysterious past together. Take a look at this revealing banter between the two lovers:
BEROWNE: Did not I dance with you in Brabant once?
ROSALINE: Did not I dance with you in Brabant once?
BEROWNE: I know you did.
ROSALINE: How needless was it then to ask the question!
Wow, Rosaline really makes a fool out of Berowne. Attempting to flirt with this pretty girl only reminds Berowne of how smart and saucy she is – she's not willing to play the role of a demure, flirtatious girl. She's ready to use her intelligence and her wit. What do you make of her demand that Berowne spend a year playing clown for sick people at the end of the play? Do you see Rosaline and Berowne living happily ever after?