The fact that I got two very different counterarguments to the complaint of Aqua not having her own nemesis sort of supports my point. On a conceptual level Aqua has less effort put into her. Terra is the young man that became Xehanort's vessel. Ventus is the boy who had his heart ripped into two and his other half is the source of the Unversed. What is Aqua? The good student and responsible one? I've seen some people argue that that's the point, Xehanort not properly accounting is why his plans ended up failing, but that doesn't excuse the neglect from a meta perspective.
What point? Her popularity? Because I never really argued against that (as it's blatantly obvious, specially in KHI. I don't think everyone needs to like her character, that will always boil down to personal opinion.), but you didn't really say anything about her rivalry with Vanitas working just fine as a villain for her rote.
Does she need a personal villain directly connected to her character for it to work? Genuinely curious here, because the only other game up to that point that had this kind of connection between hero and villain was Riku's route in CoM. Sora never had any personal beef with the Norts, and the actual villain of Days would be Xemnas & Saïx, but that game never really go into that direction.
but that doesn't excuse the neglect from a meta perspective.
.. How so? Aqua being a young girl with a big responsability having to take her friends back is the whole point, like you said, and I definitely don't think everyone needs to enjoy that and it's fine to think she's quite overrated (even if its because she merely stands out compared to the poor female writing), but how exactly is that neglected from a meta perspective in BBS?
"Terra is the young man that became Xehanort's vessel" So was Riku, who we met before him. Terra was not really unique nor was he particularly good written, but he definitely got the least love out of the fandom. I will say that actually playing through the descent into darkness was cool, even if Terra could be a huge dumbass sometimes.
"Ventus is the boy who had his heart ripped into two¹ and his other half is the source of the Unversed²"
1 happens at the very start of the game, 2 is his own character. Sure, Vanitas is part of Ventus and should be taken into consideration when talking about him, but he was more than just Ven's other half.
Regardless, between 1 and 2 is the entire chunk of Ven's route which plays pretty much like Sora's during KH2; standart protagonist trying to find his friend but ending up entangled in the world's affairs, while sometimes getting taunted by the antagonists.
What is Aqua? The good student and responsible one?
Shes the girl who lost everything. Although she was just as young as Terra, she was called a Master and had to bring her friends back, but ended up witnessing the death of her Master (in the hands of her best friend to save her other best friend, on top of that), then lost in the hands of Xehanort.
She was the good student and the responsible one, but still lost in the end.
Also the one who sacrificed herself to bring "Terra" back, not knowing of the evil that would bring upon others.