In many of the Final Fantasies, specifically preceding Final Fantasy VII, many of them did not have a specific "main character", or rather, a story whose events predominantly revolved around one character. Although Balthier seems to contradict the game's equal share of limelight for each character by proclaiming himself the leading man, he most certainly does show his share of heroism. He designates his occupation as a "noble thief", one who steals from those who deserve it. However, the term "noble thief" is something of an oxymoron; thieves aren't known for doing anything noble-- and the thought of that seems ridiculous. At some point during the journey, Balthier's motives turn from a selfish gain to that of genuine good deeds, something that he tries cover up with his humorous and sarcastic excuses.
"I hope you haven't forgotten my role in this little story"
There are a few accounts where Balthier asks for something in return for his aid, but to his disappointment, he never exactly receives something of equal value. At the beginning of the game, Balthier and Fran join the party (or vice versa, perhaps) on temporary conditions, with his eye particularly set on the dusk shard that Vaan had stolen only seconds before he was about to do the same. When Balthier is blamed for Penelo's abduction, he agrees to rescue her in exchange for the dusk shard, that is, until it's taken away by Judge Ghis. On order for Balthier to lend Ashe his airship for the journey across the sandseas, Ashe offers the spoils of King Raithwall's tomb. Balthier imagined probably what would be hidden in just about any tomb: gold, jewels, and the like. Instead, they come across another kind of treasure, Belias the esper.
Balthier: So all this time it's been here guarding the Dynast-King's treasure.
Ashe: Not so. The esper is the Dynast-King's treasure.
Balthier: Call me old-fashioned, but I was hoping for a treasure whose worth we could measure.
On one last account, Balthier asks for a "compensation" from Ashe in order for him to accompany her to the Garif Village: Rasler's wedding ring, which she also wears. Of course, this object is extremely important to Ashe, but he would only keep temporarily as he promises would eventually give it back to her. The matter isn't brought about again until the very end. Perhaps the only person who questions Balthier's intentions is Basch.
Basch: And what is it you're after, Balthier? You're a welcome hand, and a great aid, but why?
Balthier: Worried I'm out to steal the nethecite, eh? Can't say I'm unaccustomed to people doubting my intentions. Nothing could be further from my mind. Shall I swear by your sword or some such?
Basch: Apologies. But I needed to know where you stand. Her Majest depends on you. And you seemed to have an interest in the stone.
Balthier: I'm only here to see how the story unfolds. Any self-respecting leading man would do the same.
Although Balthier makes it clear of his motives, he seems understand the gravity of the situation in becoming involved with Ashe, Basch, and the resistance. Why exactly would he become involved with the war that has nothing to do with him? His decision leads him to conclude that maybe something might come out of this, and there is. There's a hint to what Balthier is looking for when he discovers that his father, Cid, is metioned by Judge Ghis as he forcefully takes the Dawn Shard from Ashe. His interests seem to switch from some form of a reward to a suspicion of his father's current mischief. After all, knowing what his father's ambition is capable of, and what trouble he could present, he felt as if it was his duty to become involved. This is furthered by his discussion with Ashe to "cut his ties with the past", and how he must put a stop to his father's doings. By this, he's showing that he can tell wrong from right, but most importantly, he's doing the world a good for a change rather than for some kind of tangible gain.
As for his insisted role as the "leading man", who is supposedly invincible, it's quite evident that he was probably kidding. He teaches Vaan how to manage the Strahl, in case something should happen to him. Of course, he's tragically misinformed that only the main hero of the story could perform heroic deeds. Every character is essential to the unflolding of the dramatic plot in a story. To be sure, some characters may be more important to the plot than others, but even the spear carriers have their appointed place in the grand epic. Fran, who seems to be right most of the time tells him, "I'd say you're in more of a supporting role", which he immediately denies. Balthier is neither main character nor spear carrier, but his contribution is no doubt just as important as the rest of the characters. Perhaps his greatest display of heroism is his decision to destroy the Bahamut at the very end, putting his life (and Fran's) on the line. The trend of assumed deaths is continued, and it isn't until a whole year after those events that the remainder of the party discovers some truth to Balthier's words: the lead never does die, does he?
"We all live in suspense, from day to day, from hour to hour; in other words, we are the hero of our own story"
-- Mary McCarthy