I'd say you're more of a supporting role
To be honest, the idea of a romance between Balthier and Fran was something I sorta-kinda liked but was reluctant to take a stance on until I finished my first play though. Even then, being the owner of a shrine to FFVII's Cloud and having thus seen "pairings" make people go eight kinds of crazy, I was still loath to go public with my interpretation until I was absolutely sure.
...So I posted a series of distressed run-on sentences and the "D:" emoticon for friends to come to my rescue. After receiving their sound advice and engaging in a bit of discussion, we came to the following consensus:
Balthier and Fran are first and foremost introduced as sky pirates and co-pilots of The Strahl. Although not strictly business in their relationship, such a partnership says more about them than sticking them with the "lovers" label could. They do everything together; they understand and use their individual strengths off one another and share in the benefits, whether that be pirating or simply getting by from day to day.
Final Fantasy XII makes a point not to make romance a point while still keeping it in the mix. Loyalty is the call of the day, and while romance is something that can come out of it, it is not the ends to the means. In some cases -- like that of Ashe and her deceased husband -- it the romance is the means to the much more important sense of loyalty and devotion. For Balthier and Fran, it seems to be more of a by-product of their unwavering dedication to their partnership.
Balthier and Fran's relationship is unique in that it is already well-established before the player is introduced to it. They walk in on Vaan's botched burglary, Balthier with his cocky swagger and Fran striding behind him in her impossibly-high silhouettes. They exude the confidence of two people who have done this uncountable times before; each knows his or her place in this production, and could probably perform the other's part, as well. The apparent trust and simple knowing between the two is something so deeply ingrained in their individual characters that we take it for granted.

The real surprise comes when their practiced routine collapses with Fran in the lighthouse. Physically and emotionally drained, both her stoic and his theatric guards are temporarily dropped. Expressions both soft and somber, Fran suggests that he take to the skies while Balthier kneels beside her. He places his hand over hers and advises her to hold on.
In a story of freedom -- one in which the self-appointed Leading Man himself claims that no one can be trusted -- intimacy is a rarity. Physical intimacy is even harder to come by, thus making the image of Fran lifting a hand to Balthier's face and he in turn grasping it all the more striking. This is special; it is a completely unguarded and very physical expression of affection that somehow bubbled to the surface, and we should treat it as such.
The close-up and over-the-shoulder shots are very deliberate and just as unique as their content. In risk of sounding like a broken record, such a technique is a visual representation of intimacy, and the director clearly wants us to take notice. However, that's not to say this single scene should overshadow the image of the confident partners in crime shown throughout the game. It is presented as an exception to Balthier and Fran's conduct for a reason; it is meant to add another dimension to that relationship rather than re-define it.
...Since I have no other place to highlight this, please refer to the best piece of concept art ever:

That in which Balthier attempts to cop a feel and is DE-NIED.