Balthier and Fran. I think it's pretty safe to say that one simply would't be as cool without the other. Are they simply friends or is there more behind their silent communication?

She is woman of a race that is not widely known to the world. A race, called Viera, that strives to isolate itself from the rest. Fran however walk among the humes and even spends all her time very closely to one of them. A man. Balthier.
Although her presence is often silent, there is much knowledge behind it and when it is essential she will enlighten the rest of the party of clues to what their next steps should be. You can sense that living with the humes, and especially Balthier, has left it's mark on her. Once in awhile, to our delight, we hear her talk back or make comical remarks to him. Like;

Balthier: "Fran is special in that she'd deign to partner with a hume."
Fran: "Oh? Like a sky pirate that chooses to steal through the sewers?"

Balthier yet again just pointed out that he is the leading man
Fran: "I'd say you're more of a supporting role."
Balthier: "Fran, please."

Often words are not even neccessary for the duo. At many occassions we see them simply giving eachother a look, as if they know exactly what the other is thinking. Fran reads Balthier's feeling very well, but I believe it's not because of any born senses of the Viera but rather that she knows him very well.

We don't actually get much background on how they came to be partners but we can sense that they have been together for a long time. They've come to trust eachother and one would never want to leave the other's side. Whether it be love or simply companionship, these two hold a mutual understanding like no other. They have much in common, one thing being their relationship to their pasts.
Both have cut all ties to their past and there will never be any turning back in either of their cases. Before meeting they were practically alone in the world.

Nothing is confirmed to us in the game but whichever name their relationship takes, it is beyond friendship. They are family. They watch eachother's backs and one can always fall back onto the other.
Would Balthier be strong enough to do the sky pirating business all by himself? And even if he had the strenght would he have wanted to? Maybe it truly is essential for a sky pirate to have a partner or a crew.
In the official description Balthier is said to be quite a ladykiller but as long as Fran is by his side I doubt either of them need anyone else.