To end this silly debate... Nomura literally said birth by sleep features 3 MAIN CHARACTERS, this trio is different from the others, Aqua is the most popular of the three, and even tho she wasn’t as much focus of the bbs story like Terra and Ventus were, she still got her own focus mini episode Game 0.2 story for herself, after getting more attention recently, she is seen as the hero of the trio
But In the end the three are still being viewed as Mains, just look at every TAV artworks, they constantly switching who sits on the middle, Ventus in middle 50% of the time, Terra in the middle 30% of the time, Aqua in the middle 20% of the time, Aqua rarely sits on the middle the first time she is seen standing or sitting on the middle is in the 2.5 cover, even in melody of memory you have Sora Roxas and Riku the leaders of their trio standing/sitting on the middle in their home screen but in bbs it is still Ventus that is standing on the middle of bbs Home Screen and not Aqua, cause again to remind you this trio was always been about three main characters, ofc only difference is this time they decided to be original and pick the Girl to be the Sora/Hero of the Group, Aqua does represent the trio cause welp she like Sora and Roxas is being treated as the Main of the trio, so because of that some or most will say Aqua is the main character, but to make it simple all three are important and are treated as main characters, that’s why birth by sleep story focused on all of them with each one of them being playable with his/her own story.
Anyway that’s all.