What's wrong? you don't trust her?
As early as their first introduction, we know that Balthier and Fran have unparalleled confidence in one another's abilities. Together they swoop in to catch Vaan; Fran easily steers the bike-like machine at breakneck speeds while Balthier practically lounges on its back. Not long after, Balthier matter-of-factly defends the viera's naturally heightened senses, convinced Fran will find an exit out of their dungeon.
But they're partners in pirating; that sort of shared ease under pressure is expected.
The cool exterior lends itself to them simply being partners on the business front. He does his job, she does hers, they share in their plunders, and no questions are asked. However, when venturing into Fran's native village, the remarkably personal side of their bond surfaces when the two pace ahead of the group for a private discussion.
Balthier: Making an appearance?
Fran: I am.
Balthier: I thought you'd left for good.
Fran: Our choices are few... This is as much for you as it is for me.
Balthier: Oh?
Fran: You are ill at ease. The nethicite troubles you? You've let your eyes betray your heart.
Balthier: Right...
Although the meaning of Fran's last quip appears to elude Balthier -- and most certainly escapes the player this early in the game -- in hindsight, it is as much an expression of concern over his past as his initial query is about hers.
The past is a painful subject for both Balthier and Fran; two lost souls find kinship in one another is no surprise. However, the past is also a very private subject for the pair, demonstrated in Balthier's indulgence in his theatrics to cover up his roots and Fran's stoic... Fran-ness. Neither is the type to divulge their personal history to just anyone, and said history doesn't lend itself to be trusted to anyone period; Balthier especially is always quick to remind his companions that one "can never know another". Nevertheless, they have confided in one another despite all reason--each taking the confession to heart and expecting the other to do the same.
The ambiguity of the discussion suggests that there are still some gaps left to bridge, but there is a mutual understanding that they are in this -- everything -- together. They're two of a kind; this particular return to her past is both as painful and as necessary for her as it is for him, and they'll stand together through it