if something untoward should happen to me, you're taking the strahl
One of Rabanastre's many orphans, Vaan spends his days longing for the far-off freedom of the skies. Thankfully, his friend Penelo keeps his feet firmly planted to survive life on the streets, though not even she can stifle the ever-energetic Vaan's bouts of vigilantism, and he brings it upon himself to take back Dalmasca's treasures.
When Balthier and Fran first encounter Vaan, they are more interested in his findings in the vault than they are in the thief. Vaan in turn is preoccupied with protecting his loot (not to mention his head) and wants nothing to do with these strangers. However, once Fran lets slip that they are sky pirates, they couldn't get rid of him if they tried.
Surprisingly, neither sky pirate makes much of an effort to escape Vaan and his constant questions. Sure, they're selective in what questions they address, tuning him out as they see fit, but he remains a constant presence in the party. Balthier readily shows the enthusiastic boy his airship, even going so far as to explain its inner-workings (or at least the basic controls) and promising it to Vaan should he need to do "something heroic". Balthier clearly recognizes potential in the would-be petty thief.
For her part, Fran would appear indifferent towards the boy, sensing he means no harm and perhaps tolerating him for her partner's sake. Nevertheless, when the aforementioned "something heroic" comes into play, she calmly places a hand to Penelo's back and guides the girl to her place at The Strahl's controls. Offering a few words of encouragement, Fran effectively passes the torch as co-pilot--at least for the time being.

It would not be much of a stretch to say that Balthier and Fran see a shade of themselves in these orphans of the war. Although lacking Balthier's educated upbringing or Fran's quiet countenance, both Vaan and Penelo yearn for the freedom of the skies and share a bond rivaled only by that of the sky pirates' own.