I could be looking into this too much but...
Could Aqua be the daughter of the previous king of Nohr? The previous king is Garon's brother and he had him assassinated. Garon disposed of Aqua by giving her to Hoshido. I'd like to think this is why she has a mask when you side with Nohr lol
I thought about this when I read Aqua's quote: "I am Aqua. I'm Nohr Kingdom's princess... or was, rather."
Isn't it weird that she refer to her royal status as "was"? If she was in the same place as Kamui, her royalty status is not going to be gone by just being raised in Hoshido. She also looks vastly different from the current royal siblings. well Kamui is too but i believe in the theory that Kamui isn't actually Hoshido royalty.
The only downside to this is that Aqua must be pretty young to remember when his dad was killed. But idk maybe Garon reminds her everytime she's not a royalty anymore when she was still in Nohr.
Also the previous king totally gave Aqua the fire emblem w/o Garon knowing.