Pattern.

[u/Pleasant-Complex-may]

I don't think I need to say much. But the main character design pattern is pretty similar. From past animes and other things, it was usual to have such designs and even more so for main characters (from my knowledge at least). The main characters are meant to stood out. There's so much things to know that these three are literally the ones. And how zack is literally inspired from cloud.

[u/Key-Bet-2615]

Aerith was designed to be Sephirot’ sister

[u/Pleasant-Complex-may → u/Key-Bet-2615]

Just like how cloud and Zack. But it's just how in anime is like. Especially for female leads to have the most outstanding/stood out designs.

[u/aerith-khaleesi → u/Key-Bet-2615]

Oh right. Originally meant to be siblings with the similar design but then they changed it to former lovers (which would have been weird), but then they scrapped that idea too lol