Okay, she's still a Princess. But we know that she's on her way to being more queenly than Helen Mirren.
From the way she handles things, we have the sense that the Princess might be a better ruler than the King of Navarre. She is straightforward and immune to flattery, and yet we're sure her sense of style and elegance would win Miranda Priestly's (of A Devil Wears Prada fame) approval. We imagine her arriving in the kingdom of Navarre clad from head to toe in Chanel with a fleet of dazzling, smart, stylish women in tow.
And yet, this girl has her feet on the ground and isn't afraid to go camping in the King's backyard (remember, according to the King's pact, no women are allowed in the castle).
Let's take a look at the political/business side of the Princess. Her first line indicates her no-nonsense nature. When Boyet attempts to butter her up, she replies, "Good Lord Boyet, my beauty, though but mean, / Needs not the painted flourish of your praise" (2.1.13-14). She may be young, attractive, and fashionable, but do not underestimate the intelligence and seriousness of this woman—she's a talented politician. She handles the role of a ruler (and all of its responsibilities and duties) incredibly well.
In addition to being a good politician, the Princess is practical. She comes to Navarre with business on her mind and seeks nothing short of accomplishing her goal. (She wants the province of Aquitaine, which lies in southwestern France but which the King of Navarre still controls as he waits for payment from her father.) While she lets herself be diverted by the men's flirtation, her task-oriented manner makes her a good businesswoman.
She doesn't blink when Marcade, a messenger from France, arrives at the end of the play, but immediately gathers her senses and asks Boyet to make preparations to leave. Additionally, the fact that she doesn't immediately agree to marry the King, but makes him prove his devotion, is another element that testifies to her practical nature. She knows better than to fall for some man she barely knows (even if he is a king).
She'll be a wise and kind queen, don't you think? We can already see, in her first scene, her concern for her subjects. She asks her ladies questions and listens thoughtfully. And in the last scene, while the men of Navarre have a blast humiliating the rustics and their performance, she continually breaks in with generous encouragement and applause. In contrast to Rosaline, her teasing is gentle and good-natured.
Such a temperate, even presence makes for an unusual antagonist, but an appropriate one in a story of finding the balance between knowledge and nature.