I do like the concept of making the letter something that can work for both.
If someone adopts her, he/she can take care of her with the info they can read.
But if by any chance if she wasn't adopted, she could have known in more detail about herself cause of her power(splashing it with her own water). All by the same letter.
Though it still place a question, how could she have "read" any of it if she wasn't under anyone and learned to read and write 😅