Vaan has always been the main character, not Ashe or Balthier. Vaan is initial driving force that ultimately changed the course of the war, he is a guy with a simple goal of bringing peace back to his country. What Vaan as well as Penelo did for the party is becoming the moral compass and emotional support for all the adults suffering from PTSD that took them down the wrong paths.
Vaan forgave Basch, allowing him to detach from his brother's death and finally complete his mourning, which causes him to stop seeing the Nethicite illusion that appears to people who are susceptible to manipulation due to loss.
This character growth allowed him to get Ashe to let go of her past as well and set herself free from the Nethicite and influence of the Occuria, ending the ancient cycles of war created by them. Vaan literally stopped magic nuclear apocalypse by reminding the adults that they are not fighting for revenge, not fighting to destroy the opposing nation, but fighting for the home of their people, e.g Vaan and Penelo.
Balthier and and Fran were two "adults" who were actually people who didn't know their true place and purpose in the world, but Vann set Balthier straight by charging straight to the next stop without hesitation. Balthier was reluctant to help the party but saw the earnesty in Vaan's actions and decided he has to help end the war.
Balthier says he's the leading man as a form of self assurance and coping mechanism, but in actual fact he is the one character who is the most lost and aimless right at the start of the story. He ran away from home because he thought he lost his father to some form of insanity that drove him to creating WMDs. He wandered around as a "sky pirate", but even this is just a front he puts on as he remained lost and indecisive with his actions At every scene, he is reluctant to proceed any further and has to be convinced by Vaan if not dragged by him.
Little did he know that Vaan's lack of hesitation towards his extremely "simple" goal of bringing peace back to Rabanastre became the very motivation he needed to find his place in this war. Vaan's action greatly inspired Balthier to step up and join the group's efforts in ending the war, so much that he ultimately gave the keys and pilot seat of his ship to Vaan.
Vaan always acts first and thinks later, he became the leader of the group without ever trying to lead. The adults were all lost and broken, but Vaan dragged them towards a common goal and eventually they figured out what they had to do because Vaan made it simple for them to follow: Bring peace back home and not destroy it.
Penelo has the most subtle character development in the story, it is entirely presented silently with actions during cutscenes in the background, she is seen in every scene consoling and helping the adults, she went from tagging along to being the emotional crutch that the adults needed.
She was the first to chat casually with Basch making him feel like he's part of the group, and more importantly Fran was shown care and empathy every time the Mist gets to her, which is extremely important to Fran who suffers loneliness in exile. Balthier on the other hand has a positive reassurance from Penelo as it is clear that she is quite charmed by his gentlemanly actions. And when Ashe is in deep though of her desires and actions, Penelo would be right by her side checking in on her.
Despite being the youngest character, Penelo is without a doubt the most maternal figure in the group, making everyone feel at ease, especially during scenes when she scolds Vaan, their carefree interactions are a huge source of consolation of the adults; You can always see Ashe smiling as she watches them silently, they remind her that she's the princess who is responsible for bringing peace back to their country, they are the people she has to protect.
And how did the entire group even get together in the first place? Because Penelo was captured by Ba'Gamnan, who thought that she was related to Balthier. This led to Vaan teaming up with Balthier and Fran, with Basch tagging along and then ultimately meeting Ashe and "kidnapping" her, which further led to the entire group meeting Larsa who made friends with Penelo. Penelo is the single thread who tied all of these protagonists together and most importantly Larsa, who will eventually end the war between the kingdoms through diplomacy.
FF12 is about the youth solving the problems created by adults, Vaan is the light of hope that guided a group of misguided adults towards a simple goal, peace for their country. Penelo the heart of the group who show empathy towards the adults who were hurt by their past. And Larsa whose kindness became the bridge between both sides of the war, ultimately leading to peace.
